If he’s just getting into Ibsen, I’d definitely start with A Doll’s House – it’s the most straightforward of the bunch. An Enemy of the People is good too, though – can you tell us which four plays are in the book he bought?
@Jeruba has a point about the editor’s choice of arrangement – the first play in the book is probably a good one to start with. Most volumes of Ibsen’s plays start with Doll’s House, Enemy of the People, or Ghosts – Ghosts is pretty heavy, though, so I wouldn’t start him with that one…
If he’s really interested in Ibsen, I’d definitely recommend that he find Brian Johnston’s translations of the plays. He’s one of the world’s foremost experts on Ibsen, and I’ve found that his translations are much easier to read and more interesting than most others – I think because he keeps the flavor and spirit of the plays as they were written, whereas some other translations fall a little flat.
(And if your son finds that he’s really into Ibsen, Brian’s also written some really interesting in-depth articles and books – Text and Supertext is particularly interesting – but only if you really like Ibsen!)