Well I can't talk about a 4 paladin + healer team, but you can see my mixed team in my sig.

I recently switched from using my druid as a healer to using my priest (specced holy), and I must say, although I loved the hots of my druid for heroics my priest does the job better.

This isn't because of tank healing, both could keep up my tank quite easily, but where a holy priest realy shines is group-healing.

For group healing with my holy priest I realy love the glyphed holy nova, its so good (even after the glyph was nerved) that some fights get realy trival (like the elemental/kollosus in Gundrak or Novos in Drak). Other than this, the priest brings Prayer of Mending to the table, which I rarely use (because my tank often is the only one in range), but for your 4-pally-group it is quite a nice tool

And don't forget, that a priest can supply a -10% physical dmg buff via Inspiration, which a druid can't.

So all in all,I never regret changing my druid to moonkin and my priest to holy, as it improved my performance in heroics quite a lot.