MY 2010 polaris 850 4x4 only works when in reverse

make sure the shift rod is not bent and is fully engaging the forward gear, may just need adjustment.

i would check transmission linkage first.might can see how to shift into forward manually,then check for linkage adjustment.

you have a reverse safety like on riding mowers with deck engaged push and hold reverser button this link showes where and how it works --against bypassing a safety only video that showes the safety clearly 1995 Polaris Magnum 425 ATV Others Disable Reverse Override Function

Hey Wayne... Does it run fine in forward? Try to eliminate anything that causes this issue in forward and reverse. Clean and set carb (by a professional).. New air filter. New spark plug.

and...
http://www.polaris.com/en-us/atv-ranger-rzr/owners-manual

1. Place the transmission in first gear

2. Rotate the lockout switch forward to unlock reverse gear

3. Shift into reverse gear

When you shift back out of reverse gear, the lockout switch will reengage automatically to prevent accidental shifting into reverse

best guess, this belt broke
http://www.ebay.com/bhp/polaris-sportsman-700-belt

The front end works off 12volts.If you have moisture in your dash your dash may not let your 4 wheel drive work and it could also not let your atv start if it doesn;t every start unplug your dash but your dash has to work to have 4 wheel drive.

if thats a cvt model maybe your shift lever is out of adjustment

With the engine NOT running, have a friend ROCK the bike forward and backward while you sit on the bike applying pressure to the shifter

The over RIDE button is not used for shifting.
it is used to over RIDE the REV LIMETER while in reverse ( for applications such as plowing)
shifting into 4wd while moving is safe on some vehicles, but can cause catastrophic damage on others.
be ABSOLUTELY sure your vehicle is designed for SHIFT ON THE FLY .
If you aren sure, then come to a complete stop befor shifting.