If you have a look through the Ilford literature, there is a PDF on basic printing. In this PDF, there is a table of what contrast you should achieve from a colour head. Without looking at the document now, I am pretty sure it suggests around grade 4.5 for an LPL. Obviously, there is going to be some variation, so to me, 4 is not too far off the mark.

EDIT - this is the document I was thinking of

According to this, 4 1/2 is around 140M. 170 wouldn't get you 5, though