"Fours" is the name used to refer to a certain type of rim (hubcap), made of wire, in 1984 (where the "four" comes from). They have a fairly distinctive shape, sticking out from the wheel.
The rims are now extremely expensive and rare, having been discontinued. This type of rim is especially popular in Houston.
The term fo' fo's was popularized in the song Still Tippin by Mike Jones Feat. Slim Thug and Paul Wall. Tippin means to cruise from side to side in your car, so if you're tippin on fo' fo's, you're cruising around on your really expensive, rare wire rims.
"Still tippin on fo' fo's" (chorus from Still Tippin' by Mike Jones)