It is often questions about if you should buy a full set, or if it is ok to buy just two tyres, to replace the ones that are most worn out. The answer is always that is better to buy a full set, as your driving performance will be better and you will be safer. To avoid having different wear between back tyres and front tyres is to rotate the tyres, every 6 months, or every time that you change between summer tyres and winter tyres. This will make sure that they will wear out more evenly.
When you mount different worn tyres between back and front, like a brand new set and a few years worn tyre, the grip will be different, which can easily result in sliding. You can easily lose control when you turn or corner. Therefore it can be dangerous to only change two tyres and it is always recommended to change the full set at once. So make sure that you will be rotating your tyres to ensure a more even wear so that when you need to buy a new set, you can buy a full new set of tyres. Rotation will then save you money as you can wait until they all are close to the recommended minimum tread depth of 4mm.