Exchange front wheel with rear wheel (Matrix 750) on Trek 4500 mountain bike?
No, not possible. 

Front wheels are front wheels and back wheels are back wheels. 

The gears are a cassette of cogs that slide onto the freehub which is the bit that lets you go forward when pedalling and coast when not. You can’t flip a back wheel in the drop outs. 

“Some wear” is normal on a trail bike with rim brakes, but your local bike store will be able tell if it’s too far gone and advise you if you need a new rim or possibly more economical a new wheel. However rims are very durable and should last at least a few years even with grindy winter mud sessions.

Also the spec claims the hubs are disk ready. If that’s true and you can afford to spend a little money you may be able to upgrade to disk brakes. 

Ask a bike mechanic first.