Seems the E-420 can take up to a 32GB Compact Flash card, (if you could afford one!), so your 4GB should be OK. It can also use an xD card, (at the same time, even, with a 'Card Selection' option to choose which card you want to use).
May be stating the obvious, but has the CF card been formatted on the camera? If it's been formatted on a computer, it would have to be reformatted in the camera before it'll work
Beg, steal, or borrow another CF card, format it, and see if that works