Can you connect 4 access points to an AirPort Extreme . Currently the 4th device goes to half speed.

We have installed a network using Mac mini server and 2 AirPort Extremes with 3 airport express. There is a master extreme and all of the other devices are bridged.. All of the devices are wired through a net gear switch. System works awesome. Except for the last access device goes to half speed.. I am not a network person but the system is so slick it just would be good to find out if there is a setting I am not seeing to allow additional access points a rated speeds. Thanks in advance for your assistance.