You can add an SSEA rating (Simple Single Engined Aeroplanes) which will allow you to fly a 4-seater. Expect another 8 hours training or so (legally a lot less, in reality 8-12), a skill test, and one written exam.
Although, most 4 seaters are actually 3 seaters, with insufficient spare payload to put anything on the 4th seat except for your jacket and flight bag!
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Last edited by Pilotage; 23rd September 2012 at 20:07.