Thats a good question Bronagh. There are a few ideas but its probably because of balance – its easier to be balanced and more agile on 4 legs – think of a table or chair. Humans evolved to stand on 2 legs over time, because it was more beneficial for us, but animals such as dogs have more benefits on 4 legs – it allows them to run faster, to climb onto things and plenty more!
Another great question from you Bronah! Dogs are great, but could you imagine what they would look (and move!) like without all four legs? Actually, most mammals (dogs are mammals, and so are cats, elephants, us, etc) have four legs. They need these legs to move and they also need them for balance. A dinner table is a good example. Most tables have four legs, so they’re nice and sturdy to eat on. Now imagine the table with only three legs…. it might get a little wobbly, but as long as you don’t push on the wrong part, it would collapse – making a total disaster of your dinner! Imagine the table with only two legs or one leg… now it just tips over. That’s the same idea for any mammal, the four legs provide dogs with the support that they need to stand and move without falling… and even run very very quickly. They evolved this way in the wild so that they could catch their food – four legs can move faster than just two (compare our running ability to that of our four legged dog friends, also horses, cheetah, etc, who use all fours to run). Hope that helps 🙂