This is for a postgraduate course, presumably? Did they give you any guidelines as to when you might hear back? (I ask as undergraduates can wait months to hear whether an application via UCAS will result in an offer, so in that context 4 weeks isn't too bad.)
If you opt to contact them, I'd word it not so much as, "Have you made a decision yet?", but more a "I realise you will have many applications, which you will want to study carefully. Do you have any guidelines, based upon past experience, as to how long that process typically takes?"
The latter is essentially asking the same question, but is less confrontational than the former.