In South Africa we would easily drive 8 hours between cities
Oh, you can easily do that too on icy roads. As long as the cities are no farther than 100km apart. ;-)
Kidding aside:
I would really love to see Paris and Prague and the Alps. And we have
a 15 old son who wants to visit a ski resort for 2 days
Well, now we're talking!
Three stops within 15 days, that should work out quite well. Right now it looks like it's four stops, but you can easily make that three. "Alps" and "ski resort" go together quite well as only one stop, don't you think? Or is there any particular reason you need to go to all the way to Slovenia to find a resort when the Alps are full of them?
Here's my suggestion:
Fly open jaw, either flying to Paris and returning from Prague, or vice versa. And in between, use trains as much as you can. They are, on the average, your most reliable means of transportation that time of year. Make sure you get reservations for window seats so you can enjoy the ride as much as possible.
I don't know how much experience you have driving icy and snowy roads, but even if you do, it can just really slow you down a lot. And I can hardly imagine you are going to enjoy driving major parts of your trip at 20km/h.
Spend four or five nights in Paris, then go to the Alps from there. Look for some nice resort either in the French Alps or somewhere on the Northern Side of the Alps. Just make sure you avoid having to cross the Alps. Then move on to Prague, and spend four or five nights there.
I personally would feel that that's quite enough for an enjoyable vacation, and I am sure Paris and Prague offer much more than you can see within four days. Plus, you can always add day trips from there.
If you feel that you absolutely must add another stop, have a look at your route once you have decided on a ski resort. And then add a place that's along the railroad route or within easy reach from there.