Well the wheel bearings are held by their own nut and locking pin and the "jeezes clip", but they should be protected from dirt getting in which is one of the functions of the cap.
I would see/ask if installer sealed the back of the rim so dirt can't get in between rotor and wheel, and that the new cap is sealed to the wheel so dirt can't get in that way.
Here is a look at whats inside there: Automatic To Manual Hub Conversion
The regular 4x4 cap fit behind the wheel so it sealed the bearing front and back, you need to do the same.
I would install manual hubs back on there, it really isn't that expensive to have wheel center drill out to fit 4x4 hubs, very common practice.
And yes you don't need 4 wheel drive....................until you do, lol.