Card, Bark, Mark, Lark, Lame and so on..
skin
ante, ant, an, a
A four-letter word that means "only" or "single" is "sole." It can refer to being the only one of something or to indicate uniqueness.
sass
If you mean the letter "u", I would say Utah is a good bet. But if you mean the word "you" then your is the only one I can think of.
Thousands of them. harp, myth, cold, etc.
made is a 4 letter word. -.-" I don't mean to sound rude, but, made is a four letter word and the only four letter word with the four letter word 'MADE' in it is made.
Mast(like on a ship) can become mask
lone
Because there is one letter in "i" and 4 letters in "love " and three in "you"
rock
One such word is ears.