Bring the rows low 1 at a time.
Read the columns Back
---CCCC
---1234
R1-00X0
R2-0000
R3-0000
R4-0000

EX
P1 7,6,5,4,3,2,1,0 = R4,R3,R2,R1,C4,C3,C2,C1
P1 = 07Fh // Set Row 1 low

if Low nible of P1 =0Fh // no key pressed
else some thing is.
in this example 02h for a scan code of 082h

Note if you did not use a diode matrix, you can get ghost keys.
This is when 2 keys are pressed at the same time. The scan may indicate a key is press when it is not.