If I buy the apple 7 watch can I use my apple 4 accessories

Yes, but I assume your series 4 watch comes with USB-B connectors and the series 7 watch comes with a USB-C connector. You can still use the USB-B setup - it will just mean a slightly slower charging rate. Since I charge my watch over night, every night - that really doesn't matter to me.

In addition to the charger comment from javaliga, if your Apple Watch Series 4 is 44mm, you can still use the watch bands you own on a 45mm Series 7 or if a 40mm Series 4, the same bands on a 41mm Series 7.