The Penn 4/0 was introduced in 1938, in my opinion it did not sell well because of the $17.50 price, this was a expensive reel during the Depression and there were many other alternatives for the saltwater fisherman for this size reel at a much lower price. In my search for first generation Penn Senators, the 4/0 has been one of the most difficult to find.
This scan is from the #6 Penn catalog which is the 1938 issue:

Last year I bought a large collection and in that collection was a early Penn Senator Lighthouse box for a 113--4/0, the box was empty but I knew where the reel was to fill it. A close friend and fellow Penn collector had found it on EBay recently and won it at a under $50 price but he would not part with it. I offered triple what he paid but there were other things he wanted to trade, so I had to find something that would soften him to a trade. Eventually I found a mint, first generation Penn 9/0 with the hard to find Rosewood handle and that inspired him to make the trade with me.
Now they are together again. A interesting point is that in 1938 the 4/0 was introduced but also was the Lighthouse box. So this pair has more than one first:

There are a number of ways to identify this early Penn Senator 4/0, the most obvious is the handle. That is what stood out in the EBay listing but the logo is also unique to the early 4/0, as is the configuration of the side plate. This early Penn has the double bar logo, it can be seen in the catalog drawing and here it is on the reel, the handle is the hourglass shaped resin type handle and the bridge assembly is mounted forward of the vertical certerline of the side plate (only early pre 1948 Senators are built this way):

There is more. The spool is German Silver with chrome plating and the spindle is drilled. Another sign that this is the earliest design.

But lets keep going. The first type clicker button Penn used on its production reels was the waffle style clicker button. Except that Penn did not use this style on the Senators. When the 9/0 was introduced in 1936 it had a Hershey Kiss style button (the first showing of this button on a Penn reel as far as I know). As far as I have found the 4/0 is the only Senator to ever have used this style clicker. It looks to me to be sort of out of place on a Senator but here it is:

And lastly, of course there are no part number markings on the reel and the bottom of the stand is blank. This is how the earliest Penn reels were all made. In the late 1930's the line capacity was stamped on the bottom of many Penn reels but this 4/0's stand is blank. Another clue that this is a very early 4/0.

So there it is John. My estimation is that it is a 1938. Am I 100% on this

The added attraction to this find was that the reel is mint. Why it is I do not know, but it is. I suspect this reel to have sat on a shelf somewhere for a very long time. it is not a ""10"" but it is very nice for a 70 year old Penn.
Early Penn reels are very special, there are thousands of Penn reels on the market but very few really special ones. In my opinion, this is a milestone Penn reel.
I hope you all enjoyed my post.