MD04 Enhancement to add button & collumn

Hi Experts,

I want to add new button & column for enhancing the MD04 screen.

User exit

EXIT_SAPMM61R_001 & EXIT_SAPLM61R_001 can be used for this. Now in my scenario, this user exit have already been implemented.

Also all the 3 buttons available for customization for the use have been utilised.

Can you please tell me if addition of  4th user defined button & collumn is possible & pointers to add the same if possible.

There is option in the menu of MD04 output screen-> Settings-> User exit Periods-> 3 Definable buttons.

Can you please tell me how Can I use those..

Thanks

Vinay