I want to display three or four lines before and after a certain string in a really huge file, which I can' topen with vi. How is this done?

I tryed

grep -i -n -r 'mysearchstring'

but this only gives me one line. I need some lines before and after to get the context!

Thanks!

I once have seen a terminal command, that does that, but I forgot and I can't find it again. :/