4th viewport window created does not work

This is very strange,Every time I create a 4th viewport window on a layout the last viewport I create prior to the forth one malfunctions(does not show anything) Its like if I could only have three working viewports in my layout at a time. I hope someone might shade some light because this is very frustrating, pleaseee, pleaseee:oops:

You may have the system variable MAXACTVP set too low. Type the variable in at the command line and set it to the max of 64.

Thank you very much ReMark, that solved it