The 4# cap should be OK. Combined with a 160 degree F. thermostat, it would be unlikely to get hot enough to pressurize. Just be sure the radiator is clean, and the water pump is working properly.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!