The OP said they were building up to 4oz with copper foil on top of the PCB (well
that's the only interpretation I can make of
I am planning to give the cooling through PCB on both top and bottom layer using stitching on 4Oz copper on both sides
And a good point about explosions - always wear eye-protection when handling power
electronics, devices fail by exploding violently.
[ BTW a single device will be enough if its something like this:
http://uk.farnell.com/ixys-semiconductor/ixfn140n20p/mosfet-n-sot-227b/dp/1427319
]