4 bar and 330 CFM gives about (4+1)*330=1650 Normal CFPM.
Take a look at the table here:
This table is made for 7 bar. So for 4 bar, we need to multiple the ncfpm by 7/4=1.75.
1650 * 1.75 = 2887.
If we look at 2887 ncfpm, we find about 100 – 125 mm pipe size, depending on the legth of the pipe.
That answer seemed a bit high (what are you using it for??) IF the 330 is already normal cfpm, look at 300*1.75=525 cfpm… which gives us about 63 mm pipe.
For an example of converting air flow to Normal airflow, see this page: http://www.air-compressor-guide.com/converting-nlmin-to-lmin.html