BCS: Mostly good info, but:
The DPP44 WOULD handle his 5 tuners. The 921 that is replacing an old tuner only needs one port when used with a DPP Separator. That leaves 3 Legacy compatible ports - and maybe saving buying 3 DP Adapters.
Also, don't forget about the cascade outputs of the DP34 - a DP21 on the cascade might serve for the fourth box.
We just don't know the whole situation. Where o' where is the OP now?