I'm old enough that I expected "fourpence" to be accepted. (But not old enough to expect to be able to say "fourpence three farthings".)
Absolutely, fourpence is correct in my book.
Is it exeptable to say "p", like in English?
No, ceiniog does not seem to be abbreviated like that.
thought 4 was pedwar
Welsh retains feminine forms (like 'un' and 'une' in French), for the numbers 2,3,and 4 (dwy, tair, pedair). Ceiniog is a feminine noun so that gives us 'pedair ceiniog'