Stock BS is generally 5". 4.5" does not create enough clearance (a 1/2" does is not enough for an added 3" of tire width).
In my opinion, 4" BS is ideal. It creates plenty of clearance for the wider tires with a wider stance; however, the tires can still be tucked under larger flares if needed.
Some people go with a 3.75" BS, but they stick too far out IMO.
4" is the sweet spot.