[JRTJH]

You have three tires that "call for 50 PSI" which means they are "6 ply or Load Range C" tires and one that "calls for 65 PSI" which is a "8 ply or Load Range D" tire.

From the "mixed heritage of tires" on your trailer, I'd suspect that you also have a variety of tire manufacture dates. Age is just as important as brand, load rating, recommended tire pressure and tire condition.

I'd recommend that you first determine the correct size tire (especially the load rating) that is on the tire placard placed on the forward roadside corner of the trailer by the trailer manufacturer. On some trailers, that information placard is located on the tongue A frame member.

If that placard is "missing", then I'd resort to trailer GVWR, axle rating, wheel rating, number of lugs on each hub/wheel and get a good feel for what weight each component is rated to carry.

It sounds like you probably have either 14" 5 lug or 15" 5 lug wheels, which typically are used on a 3500 pound axle and are typically equipped with Load Range C (6 ply) tires that carry 50 PSI.

You can verify the tire age by locating the "4 digit code" molded into the tire sidewall. It will be 4 numbers, like 4207 or 2619 or 1114. The first two numbers are the "WEEK OF MANUFACTURE" (01 through 52) and the second two numbers are the "YEAR OF MANUFACTURE" (70 through 22) indicating 1970 through 2022).

Most tire manufacturers recommend that tires "age out" and need to be replaced somewhere between 3 and 5 years from date of manufacture. I'd recommend starting with 5 tires, (include the spare) all of the same brand, sized properly to carry the trailer GVWR with a "combined tire rating" that provides about a 15% reserve capacity.

Yes, it's pretty easy to do is you can find the tire placard on the trailer, but if you can't locate that information, then it's quite a bit of "calculating and researching" to do before buying a new set of tires, but is "work worth doing to get the correct tires".

If you're "overwhelmed at this point", then provide the forum with more information about the trailer and someone will give you their perspective on the correct tires. You'll need to provide the trailer GVWR, axle ratings, number of lug nuts on each wheel and wheel size/weight rating to get started.