You can place as many in series as you wish, all I am saying is each pump add's less head, there is a limit to how much head pumps in series will produce.
4 Mags?
Why don't they just try one, then two and observe temps?
If you want that much head / flow, there are much better pumps out there for this range. Two of the new D5's will do well.
I think there wasting there money, but if you want to spend that much for an extra 0.5Deg C, go ahead.
Would be cheaper to buy an air conditioner and sit it next too your Rad
(Keep you cool in summer as well)