Thomas, just to quickly break it down: 4.2% BF is probably leaner than Usain Bolt. Even 8% would probably have you looking totally-ripped and ready for a photo-shoot as a male fitness model.
If that's the kind of body you have then not only can you gain weight, it would be hard not to gain weight unless you have some sort of parasite. Bear in mind that fitness models don't generally retain that frontpage look year-round. Typically, or so I've read, they usually have to slave away to achieve that sort of look for days out of the year, weeks at best.