This is the 2nd roll of fp4 I developed recently.
I reduced the amount of rodinol to about 1:35.
I think it reduced the grain somewhat but i cant tell for sure.
I usually process FP4 in Rodinal at 1:25, gives very crisp negs. I doubt that diluting more will improve the grain size, just the nature of the Rodinal beast. ID11, will give you fine grain but not quite the "snap" that Rodinal gives.