Yes and no. Respectively. Yes, 3 or 4 counts as a critical success on defence. But you are rolling against Defense, which is 1/2 your Parry plus other PD, so 15 Parry won't, on its own, mean 5 is a critical. (If your Defense were 15, then 5 would be a critical: but that's nearly impossible with a decent GM).
Edit: Thanks to @Yolgie, whose research means I can explain what I meant. A defence roll of 5 is a critical miss if and only if your effective skill is 15. At (3+ 1/2 skill), this means you need a weapon skill of 24, which is way beyond a normal human being; if you are playing Conan the Barbarian at the height of his powers, I imagine the GM will already have some special rules for combat.