You can easily get surplus tires to keep the trailer as is.
Though swapping over to a 5 lug design would be nice to have spares on your trailer, your stock tires are way too small for that size trailer.
For what it's going to be used for, get lug adapters and be done with it EASY!
airbagit dot com has them.
First example I found. Bolt on and go.
Also keeps the resale on the trailer as a military trailer.
And again, surplus tires are CHEAP. Just don't buy the exact tire new, they are too much money.