Capacity of PM box has been reduced?

I received a PM but found my self unable to locate or open it.
(I have had to do a a pruning within my PM's to enable me TO open the relevent item )

I have then noticed that I now only see a capacity for 4 messages within each part of the Private Messages section?? is this correct?

It is.

This is not a social site - those in groups that are within the university have more as they need it, but general members do not.

You can save your PM from the system to allow new ones in.