My father got this cat from his father who owned a small sawmill.  Dad says that it was the first ones made.  It has had changes made as it was a working cat.  Wider tracks and has newer rails.  Last ran in 2009.  
 

Grandpa changed out the tracks for wide ones as they worked better in the terrain he used it in.  He had also recently (as of when he was still alive and working in his lumber mill) put in new railings.  My dad tells me the gas starter engine is in great shape.  It has not been "prettied up"  This was a working machine, compact and powerful.  It has a straight blade on front and heavy duty trailer hitch on the back for hauling.

Are there a lot of 1939  Caterpillar RD 4's out there?  By the way the id plate says 1925.

Found out that this is actually a 1936. Grandpa had said he got it the first year this style of caterpillar came out. The little plaque actually says 1925. Have been getting hits on Facebook Market Place...my brother pointed out that those people are most likely want to sell it for scrap metal. He also believes that a good mechanic could probably have it running in an hour or so. What should I do?

Hi, Taynton.
That 1925 at the bottom right of the serial number tag refers to the year that the Caterpillar Tractor Company was formed by the merger of the Holt and Best companies.

Just my 0.02.

Good to know that! Still think it might be a 1936 though

The small number at botton of serial number tag is 1B1925, which is the part number for the tag, has nothing to to do with date.
The serial number is 1937 build date.
Details of tractor are not consistant with serial number. It has the larger sprocket hubs that were introduced in 1945 with tractor serial number 2T8270 and was used thereafter on 5T, 6U and 7U D4s.

Hi, brian18a.
Thanks for clearing that up. I could not and still can.t see the "1B" on my laptop. I'm going in for cataract surgery for my right eye soon.

Just my 0.02.

It is very possible it has different year parts on it. I mentioned above that it was a working cat on my grandfathers' lumber mill. I know for sure that he replaced the tracks to wide ones. I am sure he did other repairs as needed with parts available. He was a very good mechanic. If you feel this makes the cat worthless, I am sorry. 

So, I guess the dates are 1939 with parts from other years/models

most old cats are a conglomeration of parts. it doesnt matter.
there was also several different options available vis the dealers bassically you ordered your machine how you wanted it. originally cat only made the crawler itself. blades rippers toolbar etc where all add ons from other companies then they started making their own attachments. letournou and laplant chote are two of the companies that made attachments for cat crawlers. trackson is another company they made loaders based around cat crawlers eventually cat bought the company.

perkins engines where a supplier to cat for decades eventually cat bought them from massey-ferguesson.

Hi, Taynton.
NONE of these old Cats are 'worthless'. Even the 'dead ones' which will no longer run can be either 'resurrected' or dismantled for parts to keep others running. The simple fact that yours appears to be a 'hybrid' does not make it worthless but rather a testament to the person or people who spent time and possibly some swear words in efforts to keep it a running and useful machine.

Just my 0.02.