I currently also have a Samsung Galaxy S4 phone. Previous posters are correct about the older OS on these phones being more vulnerable to security issues. Another issue that I am having is that a number of android apps do not work on my phone as they were written utilizing a newer OS.
I have been looking for a replacement but the Galaxy S8 is too expensive. One of my sons got a Galaxy S7. The front has gorilla glass but the back of the phone does not. He dropped his phone and the back is now cracked. The Galaxy S6 does not have SIM card capabilities and the Galaxy S5 is not much newer than what I have.
I looked at the LG G6 but it has a built in battery that is not replaceable. And the Google smart phone (I can't remember the current model name) again has no SIM card capability as they are pushing their cloud storage. Motorola looks like a good alternative if I can get a decent price on a current model.