Yes, 5G Mobile Phones Will Support the 4G Network. You can use 4G SIM in your 5G Mobile Phones. All the 5G Mobile Phones come with the compatibility to support previous-generation networks like 3G and 4G.
If you have a 4G SIM Card and want to purchase a 5G Mobile Phone. Then don't worry. You can purchase that and can use your 4G SIM Card in your 5G Phone.
5G mobile phones have some extra functionality to support the 5G Network. But they don't obsolete the old generation networks.