Dear jan
Do you want find pressing 0,1,9,B only
or pressing any four or more keys
If your keys was only 4 and there are not
in one column then it is very easy

I assume port 0 is reserved for keypad
and row0->P0.0,.....,column3->P0.7

so do following steps:
1). write a code to find if any key is pressed
(if not return,else go to 2)
2). send P1,#11111110b and compare P1,#111101110b
(if equal go to 3 else call usual keypad routine)
3). send P1,#11111101b and compare P1,#011111101b
(if equal go to 4 else call usual keypad routine)
4). send P1,#11111011b and compare P1,#10111011b
(if equal go to 5 else call usual keypad routine)
5). send P1,#11110111b and compare P1,#110110111b
(if equal go to 6 else call usual keypad routine)
6). desired situation

Mehdi