After four years of use, I would start by having a look at the wheel bearings, particularly if you have taken it through deep water. I found water in one front hub on my 4X4 when it was near new after a VHC trip that involved many water crossings. The quality of the seals on cars is usually higher than trailers yet it still got in.
It is easy enough for me to say just remove the bearings then either check and repack them with grease or replace them when I worked for 19 years as a mechanic in my younger days. It is not all that easy though for someone in your position. What I would suggest you do is enroll in a basic car maintenance course. That will help you considerably with your car but while there, ask a lot of questions about your trailer, particularly about setting bearing preload.
One tip with wheel bearings, either on your car or trailer, is do not use high temperature bearing grease. It is bertone clay based and its main feature is it does not have a temperature drop off point where it will stop lubricating. This is great if you drive a V8 supercar with its brake discs glowing red but it has no water washout resistance. Use a lithium based wheel bearing grease. You will most likely find that type of grease is specified in you car owner's handbook and it will be ideal for your trailer.
