4xDPs to DPP44 then DP21 will combine one output from DPP44 and 5th DP to _one_ receiver I would say your 6000 ( after other DP-legacy adapter) will see first 4 sats from DPP44; and will be waist of DP21 and DP adapter, because DPP44 respond to 6000 as legacy SW64+SW21
Smith is right, the legacy receivers see a SW21/SW64 cascade when they look at a DPP44. This is the most complex switch combo that is supported by legacy software. There is no legacy equivalent of a DP21/DPP44 combo so it either will see the first four or more likely some invalid combination. Note that to even try the DP21/DPP44 combo you would need a legacy adapter as well.