New back tyre
 

So, the rear tyre on my road bike has worn down so wear indicators getting hard to see so figure soon be time to change plus lots of little nicks in tyre so it's going to need changing soonish. Currently have Continental GPS 4000ii on front and rear wheels. Now, I'm quite happy with the 4000s but do Continental still make these? I'm in UK and doing quick google search it seems all stores online out of stock or is it that these tyres not made any more?
I know the GPS 5000 are the newish range but wondering if 4000 still made. If not, what's thought on new 5000 on rear but keeping still good 4000 on front? Probably not great idea but what about putting new 5000 on front and putting old but still good 4000 from front onto rear?
Now, if need be I'll just buy 2 brand new GPS 5000 tyres and stick them on job done but seems such a waste and enviromentally unsound to just bin a perfectly good front tyre.

Thoughts?

If your front tire is still good, then put it on the rear wheel and put the new tire on the front wheel. I myself don't care if the tires are identical make and model. I frequently have run mixes on my bikes during all my 50 plus years of cycling. The fact that now my tires are all the same front and back is just coincidence of circumstances. A couple years ago I had a Vittoria and a Continental on my bike as that is when I last began swapping from one model tire no longer made to another.

+1 with everything Iride01 said. The Conti 5000 tubed tires are one of the best and better in every way than the 4000's IMO. The worries of mixing models/brands of tires is highly overblown if they are designed for the same purpose. As mentioned put the old 4000 on the rear and good for you for not wanting to create more waste.

Thanks guys.

Ditto!

As a purely academic point, I believe best practice is to put the lightly worn F tire on the rear and the new tire on the front. However, if you have a tire in good condition with little visible wear on the front, you are more likely to screw something up (pinched tube, displaced rim tape, damaged valve stem) by doing the swap, than you would be to have the tire fail because it is arbitrarily 'older' than the rear tire. So the pragmatist in me says, unless there is some reason you want to replace the front tire, leave it as-is under the doctrine of 'never fix a running part', and just replace the rear.

Also, as others have said, as long as the tires are generally the same size and type it will be fine. Even a couple mm difference in width (like 25mm to 28mm or so) is not a big deal, but having, say, a 38mm tire on the rear and 23mm tire on the front will likely change how the bike feels and handles - but even that you would likely get used to it within a couple rides. Also, if you have a road slick with no or minimal tread on one wheel, it might not be ideal to put a rugged knobby tire on the other wheel. Again, it will likely not be unrideable, but it might feel a little funny til you get used to it.

Both tyres will be 25mm and road slicks. This is on a Canyon Endurace road bike. I'm not racing etc, just a keen recreational rider like probably 99% of road cyclists. My concern was having the newer, better tyre on the rear which means the older worn tyre on front which menas potentially front wheel breaks away first which of course is not good and most likely unrecoverable resulting in fall.

I only ride for myself and don't do competitions or such. However I feel that I ride downhill twisty turns as fast or faster than others. You won't have the issue you are imagining even if you do leave the worn tire on the back. BUT...You will put new tire on the back, won't you?

Unless you actually ride with complete abandon of your senses and safety, then you are unlikely to have this happen. Only if you have stuff like this happen to you on good tires should you expect it to happen with bad tires too. If so then your cornering technique is lacking something.

Front tires on a bicycle generally don't wear enough to make any difference to safety. If the tire is showing other signs of age, like lots of small cuts, or dried out and cracked casing, then it is probably a better idea to put it on the rear and put the new tire on the front, but if there is no noticeable damage or wear beyond missing the 'hairs' that are formed on new tires from the manufacturing process, then there is no advantage to replacing it.