While noting this topic is being discussed in another thread too
what is your rationale to make a 4th master node?
In case of your 5-node cluster, it's certainly better to have a 6-node cluster with 3x masters and 3x data. But a 7-node cluster, 4x master and 3x data, would seem to me to add very little, if 2 masters are unavailable you will have problems.