Synthetoc oil is only recommended for 4-bangers when they have turbos. Your Camry does not.
Turbochargers, because the turbos are spun by the hot exhaust right out of the manifold, at very high speeds, and are lubricated by the oil, expose the oil to much higher temperatures stresses than your Camry engine does. Since synthetic has been shown to better stand up to high temperatures, it’s used in turbocharged engines.
Your engine does not need synthetic. He’s being honest in suggesting that synthetc is not “recommended” for 4-bangers (he’s no doubt assuming you’re talking about your Camry, which is not turbocharged). It’s simply not necessary. That’s not the same as saying that synthetic would be bad for 4-bangers, only that there’s no need to use it.
There are those who feel that “blended” oils provide better protection for all engines without the added cost of full synthetics. Apparently your mechanic is one.
I believe he’s being honest and saving you unnecessary expense.