I have a microtemp 152C 7G which is 5amp. Does it matter if I replace it with a 152C 4G wich is rated 10 amp? I don't think the amps matter because it is a thermal fuse but thought I sould ask.
While it is a thermal fuse, the low melting point alloy that causes the fuse to "open" when the current is excessive is a different size for the 5A fuse versus the 10A fuse. The 10 fuse would not operate and protect the circuit until the 10 amps had been exceeded. Since your circuit was designed with a 5 amp fuse, a fuse that will only operate above 10A would allow the current to get too high in your circuit. it is likely that the current required to "blow" the 10 amp fuse would have already damaged your circuit by the time the 10 amp fuse popped.
Probably way too late to help you, but to clear up the situation: these thermal fuses are over-temperature protection devices and have nothing to do with over-current protection. The current rating of these devices is the maximum current that they are designed to interrupt at thier rated voltages if overheating occurs. The difference in specs going from a G7 to a G4 seems to be a greater current interrupting ability and a higher temperature tolerance before the device itself fails, neither of which should be a bad thing if there is room for the slightly larger G4. The only thing I could imagine would possibly be a delayed reaction time due to the greater mass, though I would guess that to be negligible in almost all cases (as long as the device packagings were the same). You can find more info and specs on Microtemp thermal cutoffs at http://www.thermodisc.com/en-US/Products/Microtemp/Documents/Microtemp®_Thermal_Fuse_Product_Bulletin.pdf