hyphenation of phrase

"A 4-day 3-night cruise" would be better. Better yet, spell out the numerals as "four-day, three-night." Rule of thumb: spell out the numbers one through ten, use numerals for 11 through infinity.

Remember too, only hyphenate them when they are used as adjectives. A "four-day cruise" BUT "a cruise lasting four days".

This also applies to other formations: "Pick-up Sticks" (a game) BUT "pick up that stick"; "sign-up sheet" BUT "did you sign up?"