since it just a case of swapping out the crown wheel an pinion then while you have it apart to put the TT or whatever else you order in then that'd be the time to do it.

4.11's go well with 33-36 inch tyres (IMHO and as alluded to in the linked thread)
4.34's can give you a bit more pull if your higher up in tyre sizes wihtout noticabley killing your top speed
a series 4.7 ratio is going to be overkill unless you aiming for crawling speed.