The power supply components on a lot of Sony's receivers (especially the DE series models) have had problems in recent years, and some of them are known for running hot and being a bit touchy with low impedance speakers. I know that a lot of car speakers and subwoofer drivers are 4 ohms, but I don't know about DIY home speaker drivers. The protection mode does exactly what you say it does, protects the amp and prevents overloads. But, some of them aren't as well designed as others. With some receivers, the protection mode locks up the entire receiver and requires a service call to reset the unit. Others are very sensitive and go into protection mode very frequently.

Check the back of the unit and see if it has an impedance selector switch (I believe that UL now requires one for amplifiers to get their approval). In the 8 ohm position, the protection mode will engage a lot sooner than in the 4 ohm position. But, even in that more fault tolerant position, you still might run into problems if you're driving multiple speakers and some of them have highly variable impedances.