Hi Brian, I have not taken the rear window out of my R4 but assuming it is the same as a 2cv quarter window to refit, start by fitting the rubber to the window, rather than to the car. Next fit the bottom of the window rubber first to the metal lip in the body lifting the rubber where the curves start to turn up. This is important or the window will not sit low enough on the lip. Some people put a piece of string round the rubber so once the lip is started you can pull the string out and it brings the rubber over the metal lip with it. This can need lubricant to assist. On 2cv windows I pull the rubber over with a rolling motion using my thumb and don't bother with the string. Go up both sides at the same time (a little one side then a little the other etc) and don't be afraid to stop after two or three inches up to wobble the window down fully onto the metal lip.
Halfway in, you may need to place the palm of your hand on the outer face of the glass and push the window into the aperture as it will help the seal seat better round the metal lip, which in turn helps getting the rest of the rubber over the lip easier. Once at the top of the sides, continue on one side across the top, so finishing in a top corner. It is all a little about feel. From experience, 2cv side windows you can be remarkably rough with pushing them about mid fit, but the windscreens are just itching to crack. I suspect an R4 side window will be similar to a 2cv side window.
Best of luck!