How much boost can a 1991 4g63 motor take?

There isn't really a set pressure it can handle, it all depends what is done to the motor. If it's completely stock, not more than 15 psi because the fuel system can't keep up with much more. Upgraded fuel system and turbo, you can do over 20. I have a 1990 tsi with fuel upgrades, bigger turbo etc, but stock internals and I'm pushing close to 23 psi.