I've been hearing a little about multiple PSU solutions. Is this dangerous in any way?
Not if you do it correctly and the PSU are the newer/better units. Make sure the PSU connected to the same systems are on the same ground, usually plugging them into the same power strip/socket is fine.
Some PSU might not work with zero load on the +5V and +3.3V line so you might want to plug one into the motherboard and the other one to the HDD to put a load on the +5V/3.3V circuit. The newer ones shouldn't have a problem but I don't know what those PSU you have might be, better to be safe than sorry.
1x 550w PSU
1x 800w PSU
550w--> Motherboard, Card 1
800w--> Card 2, Card 3, Card 4
In this case, it might be better to mix
800W -> board, Card 1, Card 2
550W -> Card 3, Card 4
The reason is to distribute the load so that both PSU loaded as lightly relative to their max capacity.
Assuming your board draws about 100W and each card draws 200W, this would give a 500/800 (62.5%) and 400/500(72%). Although it's more likely going to be a 68%/70% thing since card 3/4 would still draw some power through the PCI-e connector, you'll have to use a clamp ammeter to find out the actual split to optimize.
Going your original route, you'll end up with 300/550 (54.5%) and 600/800 (75%) which makes the 800W operate at lower efficiency and so cost slightly more in terms of electricity.