I have an Apple tv which is over 4 years old. Will it work with the new apple tv format and its apps?

Maybe. It depends on the exact model. Identify your Apple TV model - Apple Support

Apple TV apps can be installed on Apple TV (4th generation or later). [A1625, A1842, A2169]

The App Store is not available on Apple TV (3rd generation or earlier). [A1218, A1378, A1427, A1469]

Apple TV (4th generation) was introduced in 2015, which is more than 4 years ago.

Apple TV (3rd generation) was discontinued in 2016, so that overlaps a bit with the later model.

If the box is 1.4 inch (3.5 cm) high, then it is 4th generation or later.

If the box is 0.9 inch (2.3 cm) high, then it is 2nd or 3rd generation.

Please Identify your Apple TV model . Telling us your devices approximate age does not help us, if we know what you have then we can answer the question. If you find that own an Apple TV 3 or earlier, it will not run the same version of TV OS as the Apple TV HD and Apple TV 4K models, it is simply too old and does not have the hardware needed.