What do you do, when you notice at the end of the summer that you will need a new set of summer tyres? Some people tend to say that you can get a good price on summer tyres at the end of summer, as the demand is low and not many sellers will be able to sell any during the winter season, so they will have to carry that inventory during the next 3-6 months. So they tend to be open to make you a good offer. And as long as you store them correctly, the tyres will be good to go for you when it is time to change back to summer tyres. I would say, that if you are already below the 4mm, then you might as well buy a set right away and use them until you change to winter tyres.
If you have set a date to change the tyres to winter tyres and it is within a few weeks, then you can still try to get a good deal and buy them now and store them with your service station so that you don’t have to start looking next year for a new set of tyres. Tyres do not go out of style. There might be some improvements done between the years, but most significant advances in technology tend to be on the winter tyres. So buying the tyres and storing them for 3-6 months, will not have any negative impact, if you store them in cool conditions and out of sunlight, which both should be quite easy to do during the winter season.
Key for safety is that you change tyres while you still have sufficient tread depth, to ensure that you have adequate performance, that you change tyres to winter tyres early enough to ensure that you remain safe during the onset of winter and that you have good quality winter tyres. All these factors might sound not too critical, but they all contribute to your safety and should not be neglected. The winter conditions are that challenging to drive in especially compared to the summer season. Remember that compared to your summer tyres, you will need to count on much longer braking distance, so keep your distance to the drivers in front of you.