I know that separately they should mean something like "beauty", "love", "friend", "mad", but written like this, do they form a sentence?
Let's slightly analyze them:
According to: http://xh.5156edu.com/
The following characters falls in these parts of speech:
美:【形】 【動】 【量】 愛:【動】 【名】 友:【動】 【名】 狂:【形】 【動】
Since these characters would definitely not make a valid sentence in Modern Chinese, let's assume we are dealing with Classical Chinese.
A typical Classical Chinese sentence consist of:
主語、謂語、賓語 or you can understand it like
Subject Predicate Object in English.
This is a possible combination:
We can do a 詞性活用 (a hack if you will), and make 狂 become a 【名】 to mean "a crazy person". The sentence would mean:
Beautiful love befriends with a crazy person.
Please note that this is just for the sake of answering the question. Don't use it because nobody will understand it.
Unfortunately I don't think these four characters form a sentence or phrase. My guess is this comes from someone's tattoo because these four characters are very common in U.S. But I have never seen them appearing together...
No, they don't form a sentence = = They are just four random words