I say go for it

The fresh air vent is in the front end of the crankcase left side thru the oil filler pipe, air cleaner shutter works thru the air cleaner assy.
Hope you can see the bolt head back there, it is with sliding this bolt head that you open or close the ventilation, I would guess that as mentioned Plymouth does not have the option of opening or closing, plymouth is just always open.

I may have a 29 or 30 Plymouth instruction book if you do not. Let me know. My 29/30 Dodge instruction book reads as follows ( and again I am only assuming this is the same deal, I know very little about Plymouth other than I would like to own one )   Crankcase ventilation.............
Due to natural operating condition of a gasoline engine, gasoline and water vapors form in the crankcase which under certain conditions may cause an etching of the brightly finished steel bearing surfaces in the engine and dilution of the crankcase oil. These gases are expelled (before condensing ) from the crankcase thru the ventilator pipe connected to the intake of the air cleaner from the rear valve spring cover.............
There is an actual air cleaner shutter on Dodge and most often the very small bolt that acts as the handle for the sliding shutter is missing from these engines, I had assumed when you said 4 mm hole you were referencing the spot where the small bolt might be, it is this small bolt that again acts as a handle and when moved slides open or closed this shutter.
You Plymouth may not be designed quite as intricately as this and that would not surprise me as it was a bit of a cheaper car so this is why I would still suggest that this hole of yours is still this breather and by plugging it you will eventually cause serious damage to your engine.
Just a thought

It almost has to be your crankcase vent then and I would not plug it

That is a small hole and so close to the edge of the pan, it does not look like what I had been reffering to but I would ask other Plymouth owners to verify they do not have the hole before you cover it up

I dont know unless you post a picture if we are thinkng the same thing but it sounds like your air cleaner shutter, it is part of your crankcase ventilation and it sounds like you may be missing a piece.
It is important as it helps ventilate the crankcase

I am pretty sure I know what it is and what its purpose is and it does have a purpose and I would not just plug it  but just to be sure we are on the same page it would be best if you could post a picture

Thank-you Glen for sending these to Robert, as far as from what I have noticed in his e-mails he has a very nice car and appears to want to keep it that way

And I am not in the buisness of attemting to swindle anyone out of anything just to make a few bucks, ( isnt that what the scum on Craiglist do )  I spend my time on very few forums because I enjoy these topics and I am able to learn things and share what I learn.
The person I mentioned has a very nice original 1928 Victory car, I have been helping him with information, he asked me if I could help him with grease fittings ( getting originals ) and I said I would.
This was my response to him several weeks ago on grease fittings..............I will find one for you eventually but it will take time, just so you know Victory and Standard Six are identical  and there are many more standard six engine lying about.
Yes zerk fittings will be easy, ( trust me on this, it is how I get most of my parts ) when on e-bay or AACA find someone that is selling parts ( there is a front end on AACA now ) offer the guy a few bucks just for the fittings. Money is the only thing that motivates most people unfortunately so if you offer say 5 dollars for a few a fittings ( whats 5 bucks ? ) and explain to the guy why you want them he will not only feel like he is being offered the deal of a lifetime but it will also help justify his cutting up a perfectly good car and for a few moments it will make him feel better...................This is his e-mail address and I am sure he would be thank-full."Robert Bardin" <rbardin@me.com>

I have just had a relatively new 28 Victory owner ask if I had any to spare so I am sure we would appreciate having the fittings if they are still available

Quote.....I know what a Zerk fitting is and my 1948 Plymouth has Zerk fittings.  At least the rear axel for sure........That is interesting and I think your idea about why they may have still been used there is good. Interesting info, thanks.
Quote.......He stated that the early 28-33 Plymouths used the zerk fitting, either straight or angled depending on location. That the 34 was the first year to use the hydraulic (or Alemite) fittings...........Thanks for that Gary

Great, working on a grocery list now, thanks

That is great Dave, would it be possible to get some more details from you concerning the tools. Maybe we can exchange e-mail? What I am doing is collecting measurements and any identifying marks so that future owners can put togther their own kit if they desire.
Looks like a Monroe pump to me for example part # 12520 which may be stamped on the collar just below the handle, might also be a patent date on same collar. ( Just an example of what I am needing )
Hard to tell is that a square shank screwdriver, length would be helpfull along with a closer pict.
I am going to take a guess here and say your lug wrench is approx 15 1/2 long, can you confirm this?
Is that an Ajax jack, what is stamped into it other than maybe Ajax? Length fully closed
Grease gun, Alemite?? What is the model # and the last patent date?
Length of pump fully closed 19 5/8 approx?
Need length of crank assuming it fits your car
Hub cap wrench appears correct as well, overall length would be nice
I have been asking people to lay the tools down individually with a tape measure starting at one end as a referance point and then snap a pict and tell me measurement.
Are you able to do this? Thanks again

Here is some good general info.........The ball end grease fitting which most people mistakenly call a Zerk fitting is actually an Alemite Hydraulic fitting. It was developed by Alemite enginners in 1932-33 and was common on most American cars by 1934.

quote........My 1948 P15 has zerk fittings......This has to be a typo! or there is confusion on what a zerk fitting is. The fitting with the ball on the end is not a zerk as everyone likes to call it.