Reasons for a single loop1) Cost
With a single loop, there is only one pump to buy, maybe two for power/redundancy. With multiple loops, you will need multiple pumps no matter what. More fittings to buy, multiple reservoirs.2) Maintenance
Only one loop to build/bleed/drain/clean/monitor.3) Noise
Less noise from a single (or dual) pump over 4 separate pumps.4) Heat
Pumps do inject heat into your loop, a single pump won't heat up your loop as much as 4 would heat up their individual loops.5) Safety
With multiple loops, you multiply the chances of pump failure and leaks by the amount of loops. Should one of the loops fail, it's CPU will heat up and the rig needs to be stopped. With a single loop, there are fewer chances of loop failure. And you can always add a second pump for more power and more redundancy. A pump fails and the other one keeps the show on the road.6) Performance
Most importantly, you will get much better performance out of a single loop, especially if the CPU are unevenly loaded.. Imagine if you will 4 loops, one loop for each CPU and a 240 rad in each loop. So CPU-1 gets loaded and gets hotter while the other 3 are idling. That CPU-1 could really use more rads to help it cool down, but it's limited to the 240 rad in it's loop while the other 3 rads are basically useless as their CPUs are idling.
With multiple loops, whenever one loop is idle, it's rads are more or less wasted. But in a single loop, all the rads get to work where they are needed, when they are needed.Reasons for multiple loops1) Eye candy
You get to have multiple reservoirs with posible multiple coolant colors. That's the only pro of a multiple loop set up. And I imagine that's not a big concern in a server environment.