Question 22974
This process is almost impossible to type out and explain
on-line.  But, if you were able to do the task using minor
expansion on the 3X3 matrix, the method is the same for 
your 4X4.  Maybe if you indicate exactly where in the process
you are running into trouble you could get some help on-line
to help you through that step.

Also, the whole process can be found if you will use Google
to search for "Math World" "Determinants".

Cheers,
Stan H.