Some encounters that you can do with 5 shamans, you'll have problems with as a 4 shammy+1pally setup simply because you've sacrificed too much dps. Good examples of that could be Heroic Mechanar boss Sepethrea and Heroic Magisters Terrace boss Vexallus. When you get enough spelldamage you can nuke these 2 down before you're even threatened. I'd say 600 spelldamage is the limit to be able to do encounters like this with 5 shammies, and even then you'll still need some luck. With 800+ spelldamage on 5 shamans it becomes easy I'd say. 4 shammies with 600 spelldamage + a paladin who really doesn't bring any dps, both encounters would be very difficult. It would of course be possible (everything is possible), but with a paldin + 4 shammies you'll need to micro alot more to be effective.