No, you can't use a 5G connection on a 4G smartphone. 5G and 4G are not created with the same technology, so it will be practically impossible for the 5G network to work on a 4G smartphone. In other words, smartphones that use 4G LTE are designed in such a way that they are incompatible with the next generation of wireless mobile networks.
The technology is going to work perfectly on new phones that are designed to use 5G networks. The issue here is that 5G networks are yet to be rolled out to all parts of the world, so only a few people are using the technology.
But very soon, a lot of smartphones that support 5G networks will be made for people to buy. On the other hand, it is possible for a 5G smartphone to use the 4G network. The fact that you can easily switch to the 3G network even with a phone that uses 4G means you can equally use the 4G network on a 5G smartphone.
5G suggests a new type of core network which will probably support existing cellular devices, but this will be via existing radio. The possibility is that there may never be a 5G smartphone, as such, because 5G may be more valuable in application areas that are not yet well served. The truth is, moving up to 5G means the 4G phone should get better by improving the speed and coverage of 4G networks.
5G is the next generation of 4G technology. However, you need a network that's running on 5G to do that. 5G is about more than merely transferring GB's to and from your iPhone more quickly. The 5G revolution will cast a much wider net.
A lot of people are concerned that with the appearance of 5G, it will make their 4G phones obsolete. This is not something that you should worry about at this point in time. 5G is still in its premature stage, and even if there are now phones that will allow 5G network, it will take some time before people will embrace 5G.
The thing about 4G and 5G is that they can co-exist. In fact, the use of 5G may help improve the speed of your 4G phone. There are some aspects of 4G and 5G that are shared which explains why 4G cannot be removed just yet.