I don't mind 4", it'll give more room for tracks, which my board will have a lot of, and I might be able to make them 8 mils. 6 mil tracks don't cost anymore than 8 mil ones, from most suppliers, though.
Leon,If possible, keep it 8 mils or 0.2 mm. Many of the protoshops (especially in the cheap East) charge more of the 0.15 mm tracks. Also the yield will be better with wider tracks.
Ahti OH2RZPS: Several decades ago I saw one American engineer complaining in the IEEE Spectrum that people are going inch by inch to the SI units of measurements at the speed of furlong per fortnight. Seems to be true, still! :-D