Remember that the 4000 series runs 1/2 the speed of the 5500. Each uplink/downlink cascade cable runs at 72Gb(56xx), 40Gb(55xx), 20Gb(4xxx). Also the switch fabric speed is commensurate.
When upgrading a stack of 5500's to include a 5600 you need to:
1) Configure the 5600 first as a non-base
2) Configure the 5600 to be in Hybrid mode (this changes the cascade cable speed from 72 to 40)
3) Add the 5600 to the existing 5500 stack (this allows the switch to join the stack and be learned)
4) Take a smoke break (5 minutes max) while the base switch learns the new switch
5) Change the 5500's base switch's BASE SWITCH to non-base
6) Change the 5600's BASE SWITCH to base
7) through console/gui/etc electronically tell the 5600 to become base
optionally

Add additional 5000 switches to the stack.
The 5600's should be grouped together, then the 5500's.
If you have more than 1 5600 in a hybrid stack, Switch #1 (base) and Switch #2 (backup to base) must be 5600's. The thought process is that if the base dies, we want another 5600 to take over as base.
Cheers, Uncle John.