To sum up, you want to 4-bet to around 2.5x the 3-bettors raise when you are in position and around 3x his raise when you are out of position. In tournaments, size down to around 2.2x when in position and 2.7x when out of position.

A good winning 6-max reg will typically have a 4bet range in the region of 4-5%, whereas the average player has a 4bet range in the 2-3% region. In other words, it's not a big deal to slightly overfold when facing a preflop 4bet from an unknown.