I meant a north-south-east-west compass. Essential when hunting buried treasure. And of course, how could I have forgotten - a metal detector!! Do please get him one of those. Try Science Museum website.
Actually the other kind of compass is very popular too, as, bizarrely, is a set square, a protractor, etc.
If he says he wants Playmobil then go for it - castle pretty good for girly play too, specially if you can get a dragon or two - Early Learning Centre does brilliant ones. Also unicorns, phoenixes ... but now I'm on to Toys For Four Year Old Girls.