im guessing it's "last" (:
Some four letter words with the first letter f are:facefadefailfairfallfamefarmfastfearfeedfeelfeetfestfilefillfilmfinefirefirmfishfistflapflatflewflitflipflitflopfluefoalfoamfoldfolkfoodfoolfootforkformfortfourfrayfreefretfromfuelfullfumefuryfussfuzz
best
s
r for rich
than
s for seek
pies
Vega
h as in hunt
passpastpesopestposhpostpush
d for dawn
D for deck I for Info