DB2 zOS increasing DSSIZE vs adding partition?
As long as there is no application code that uses the partitioning scheme in their logic, adding a partition is a solution to your space issue. 

I would suggest that you use 5 partitions instead of 4. Your data should then spread out more evenly.

P1 00-20                                                                    
P2 21-40                                                                    
P3 41-60                                                                    
P4 61-80                                                                    
P5 81-100