Another Sakuṇagghisuttaṃ question

One more student question about SN 47.6 Sakuṇagghisuttaṃ (PTS S V p. 147):

Near the end of the sutta is the phrase (in the nominative case)  "cattāro satipaṭṭhānā", which has been translated as "the four frames of reference":

http://www.accesstoinsight.org/tipitaka/sn/sn47/sn47.006.than.html

I can see how there might be differing translations of "satipaṭṭhānā". But I am more concerned with the preceding adjective "cattāro", which should be simpler, because it just means "four".

The noun "satipaṭṭhāna" is neuter. But the neuter nominative form of "catu" is "cattāri". So why is "cattāro" (the masculine form) used instead of "cattāri" (the neuter form) in this phrase ?

Would appreciate any suggestions here

- Steve