As far as I know, I really don't think using a 5G Wi-Fi connection on your 4G smartphone will affect your phone in any way. But, whether you are going to have a strong signal or a weak one will depend on how strong the Wi-Fi is.
It will only be bad for your 4G smartphone if it is using 5G SIM. 4G smartphones can't run on 5G because they are incompatible with each other. 5G network can only run perfectly well if it is being used by 5G smartphones. Since smartphones that run on 4G cannot use 5G networks; however, one of the alternative ways to use a 5G network on 4G smartphones is by connecting to 5G Wi-Fi.
This will definitely work out, though the internet speed will not be as though you are using the true 5G networks. It will definitely take a while before people can start getting smartphones that run on 5G, though those who can afford are using it already.
You can use 5G SIM on your current 4G phones, and 5G speeds will be about twofold of what we are using in 4G now. Smartphone operating systems have inbuilt support for 5G, which is disabled by default. So, to use 5G in 4G, all we need to do is enable default. Enabling this will permit us to use 5G in the 4G network on any smartphone.
The remarkable thing about this approach is that we will not need any third-party tools to do this. It might work on your current 4G LTE phone if it proves capable of scaling, the cell could dramatically change the way wireless networks operate. It's unlikely that LTE carriers would replace their networks any time soon. Its compatibility with LTE shakes things up. For instance, operators could utilize cell antennas in congested hotspots such as airports, sports stadiums, city centers, and in crowded areas.
It is not possible to use 5G on your 4G smartphone, but there is no need to worry because 4G will probably stick around longer than expected. There are so many aspects of 4G and 5G that need to be shared. If 4G would be removed, this may cause further issues with the 5G.
Since the 5G is still a relatively new network, there are still some changes that need to be done to it to ensure that it will work according to how people expect it to work. The only way that you can access 5G is if you would change your phone to the new 5G-able phones that are available right now. There will still be more that will be released in the long run.