Does "it depends" satisfy you? 
really cannot know whether adding server5 to the workload will be too much for the host - it depends entirely on how much of the allocated resources are actively used by the other VM servers. What kinds of work-load are they?
What percentage of the hosts CPU and memory are still available when the other servers are running?
What about peak times?
The fact that you have allocated 2 CPUs to 3 of them suggests either that:
1)the os and applications know how to use multi cpus, and require multi-cpus to function properly, or:
2)You allocated more than each needs.
Only by watching current utilization will you be able to determine whether the additional server will have the resources it requires.