to be fair, "4" wasn't an option you had as a reviewer, so it was either 3 or 5, and 5 is "slightly below acceptance threshold". So if you feel like there is a flaw in the paper (even one you could recover from in rebuttals), "3" was the natural vote to give. Personally, for most papers I rated "3", the authors could come back from it if they manage to address my concerns properly.