Can someone please let me know what size alloy wheels are ok to fit my mk4 vw golf? will audi a4 16"?

Any wheels bigger than 15" in diameter and less than 9" wide (with an appropriate offset) will fit your MK4 Golf fine. If you want the wheels to bolt up without the use of adapters, you will need to find ones that have a 5x100 bolt pattern, and a 57.1mm centerbore (or allow the use of aftermarket hub centering rings). The Audi A4 has a different bolt pattern than then Golf. The only Audi wheels that bolt on without modification are off a MK1 Audi TT.