Post, as in postcard.
card or carl
post = signpost.
card box code haste post
card care carp cars
post
The answer is post, to make postcard, postbox, postcode, postbag, and posthaste.
Post.
A four letter word starting with the letter a is able.
Cart Colt Cram Card Cats Cite Clam Cold
card, box, code , bag, haste
A four letter word for 'in addition to' is also.
That Four-Letter Word was created in 2006.