Dana, all my packets of D-19 are too old to use, so I have not tested it with FP4+. It seems that HP5+ does not expand as readily as FP4+, so FP4+ in D-19 might have a tendency to get out of hand quickly. I'll put that test in my "one-of-these-days" file.
PS -- I have processed both HP5+ and FP4+ in Ilford PQ Universal developer and the results were remarkably different, much less expansion in the HP5+. If I were to use HP5+ again (especially for carbon printing), I'd be looking for some D-19, or making up some scratch.