John
I’ve not looked on the site for sometime, so you’ve probably replaced the side window by now. But for what it’s worth, if Starglass have both 4 & 5mm glass available (is that right, Starglass can still supply side windows for the Ace?) you would IMO be better off staying with the original 4mm. Not only is the 5mm likely to be a tight fit in the window channels making operation sluggish, but if memory serves me correct, and the rise & fall mechanism guide runner is a press fit on the bottom of the glass, you’ll almost certainly encounter problems persuading the runner to accommodate the thicker glass (forcing the runners on is not easy even when using the designed glass thickness!).
Max