Since there were no bonuses to time for melee attacks in RE4 Mercs every character was viable. It actually mattered what weapon loadouts each character had. The level designs were great, and you didn't have to break the time bonuses you could just pick them up. Knowledge of the level and how the enemies spawned mattered just as much as RE6 Mercs.
With the updates to movement in RE6, I will say that RE4 movement isn't so great in a mode like the Mercenaries. RE4 Mercs did have flaws, but I just liked actually using my guns and finding out the fastest way to kill enemies with each character.
I'm not saying I dislike RE5 or RE6 Mercs, I think they're great too, but I really had a lot of fun with RE4 Mercs.