I believe URC only recommends using 3 antennas at most on a single MRF (probably due to power limitations), but you may be able to use more if you use a seperate power supply for the last two antennas. Remember, these are 5VDC devices and you MUST use a regualted 5VDC supply.
Eric Johnson should be able to verify this.
There are 10 types of people in this world, There are those who understand binary, and those who don't!