No, there is no comparison for battle prowess between Sengoku and the admirals. The admirals being stated as the strongest in the marines is true, but Sengoku is a fleet admiral and was also once an admiral, meaning that he has also held the distinction of being known as the strongest of the marines.
Whether Sengoku is stronger or weaker than the admirals remains to be seen. We've seen a little of all four's power, but I could still not really rank them. Akainu would seem the obvious winner, but it's only partly because we've seen so much of him in this war, especially compared to everyone else.
Remember, rank is not everything. Garp has always been hinted as stronger than admirals, though Sengoku believes he's able to keep him in check. Both Aokoji and Saul were vice admirals, yet the difference in battle power was enormous 20 years ago on Ohara. Smoker is only a commander, yet he often mentions that people far weaker than him are above him in rank.
Anyway, I've got a feeling either Akainu or Aokoji will be the strongest marines of the bunch, but since it could be very likely Sengoku stays at MHQ (what remains of it anyway), we may never really get a chance to compare the Fleet admiral to the three admirals again (except perhaps in flashbacks, or unless he ends up sending himself to the New World?)