I don't know a single 16 man nim progression guild that just uses 4 healers for nim progression. Most used 5-6 healers.
What 16m guild uses more than 4 healers for progression? I believe that none does.
For a couple of fights with low DPS and high healing requirements, having one DPS spec hybrid or full heals may be safer for progression or title runs. But 4.5- or 5-healing is the exception for 16m rather than the rule.