just found this sibers, hope it helps.
I had this problem too. I spoke to Dell Support. Apparently, Fn-F4 turns on the number lock light regardless of whether or not the number lock feature is enabled.
Here is how you enable the number lock feature:
1) While booting up press F2 repeatedly
2) After the Dell logo appears, you will be diverted to a Settings menu
3) Expand "POST behavior"
4) Select "Keypad (Embedded)"
5) Press the "By Num Lock" button
6) Press “Apply”
7) Press “Exit”