Simple words:
* four
* fourteen
* forty
* fourfold
* fourteenth
Longer words:
* four-poster
* four-wheel
* four-letter
* four-sided
* four-color
Compound words:
* four-in-hand
* four-star
* four-flush
* four-poster bed
* four-wheel drive
Phrases:
* the four corners of the earth
* the four seasons
* the four freedoms
Other:
* quartet (though it doesn't have "four" in it, it refers to four people)
There are many other words that could be included, depending on how broadly you define "contain." Do you have a specific word in mind, or a category of words you're interested in?