At the start of the year your PRSI should be...
€551 @ 2% = €11.02
€4,000 - €551 = €3,349 @ 6% = €206.94
Total = €217.96
When you hit the PRSI contribution ceiling of €46,600 later in the year the amount will reduce, e.g.: to a minimum of:
€4,000 @ 2% = €80
There is no "monthly PRSI cut off point". The PRSI ceiling only kicks in on a cumulative basis if/when you hit it during the year.
might help because it seems to work out things on a monthly basis for the full year presumably mirroring what should appear on your palslips.