I have been running 2 TerraMiner IV's for a year now, Feb batch which showed up March 13th.
One of them always ran great until about last month, pretty sure its a pump, some smacking brought it back to life but it looks to have gotten worse.
The other one had 2 great boards for the first month, then one of them always gave problems.
Tonight that problematic one has croaked, only half the larsen scanner lights up, but its probly a pump issue as well.
Had to disconnect the bad board in order to get the good one up an running, also had to leave both supplies plugged in.
Out of my 8 cooling blocks I have replaced 2 of the shitloads of factory-applied thermal paste with a small amount of decent stuff.
They ran pretty much the same as before for me, possibly a little cooler.
Tomorrow I'll try carman336's theory of abundance and pump cleanout to see if I can get the dead one up again.

The only difference I may have from most here is that I run them both from a 240VAC Tripplite PDU and have had pretty great luck with flawless power so far.
I still get anywhere from around 0.02 - 0.05 BTC/24hours from them and, between power and internets, they cost about USD $320/month to run.
I look at it as a dollar-cost-averaging investment over time plus doing my part toward helping the network.

Great work carman336, hope those S4's are working out.