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