If you are going to be buying a new cell phone any time in the near future, you will not have an option about 4G vs 5G - it will have to be 4G since 5G isn't available yet. And even when 5G phone become available, they'll be so expensive, it will be another year after that before regular folks will start to consider them viable options.
As far as what the 4G/5G denotes, that's just a classification for the type of communication the phone uses to talk to the cell towers (and back to home @ATT, Sprint, etc.). 4G is better than 3G, and 5G is better than 4G. As as general rule, you can think of each successive generation as just being able to handle more data at the same time versus the previous generation. This will only come into play if you're watching videos or streaming movies on your phone using the wireless provider as your internet connection.
If that's all still a little confusing, it might help to think of the xG (2G, 3G, 4G, 5G, etc.) designations like fuel management in internal combustion engines. The old-school carb-fed engines could be thought of as 2G, and the multi-port fuel injection engines could be thought of as 3G.
The latest and greatest nowadays is direct injection, which could be thought of as 4G. I don't know what's next for our engines, but think of that as 5G - imminent but not in use yet.
clear as mud?