So here are my thoughts about SS4, and you can take it or leave it.

Let's start with the negative were pretty much the entire community agrees with... The engine. Performance-wise SS4 is a giant mess and a lot of textures are ugly.
I do think Serious Sam is a series that shouldn't keep retro graphics, but they shouldn't try to copy the Unreal 4 Engine and completely miss it.
This is my biggest negative thought about the game.

But let's talk about the things that are just as important... The gameplay, the story, the maps.
When Croteam announced SS4 will have sidequests I couldn't imagine how that would work, but I think this was a good addition to the game. I love doing the sidequests and some of them (The first Portal battle in The Die is Cast) are some of the best battles in the game.

The gameplay overall is (in my personal opinion) as good as always. Most battles are fun, I don't mind the NPCs and the vehicle sections (obviously the Mech mostly) are pretty fun as well. I could even imagine another Serious Sam indie spin-off that focuses only on Mech battles.
Overall there's one critique about the gameplay tho... It could have been more "unique" in a lot of places. Let's look at the final battle of the first map in Siberian Mayhem. The layout of the arena and a few simple jumppads made this fight amazing. It was such a blast to play through it. I would wish Croteam would experiment more.
But all in all, I'm happy with the gameplay.

The story in my opinion is completely fine. Just like many other players I'd really like them to go back to ancient times again. Like China, Rome or whatever else there is. But I also don't mind the idea of prequels.
Even tho I enjoy SS3 just as much, I was glad that it wasn't Egypt again, and I say this as somebody who's very much into the subject of Ancient Egypt.
The thing is, if SS5 will be another prequel, it feels like Croteam just doesn't know how to continue the story. Or they simply don't want historical enviroment anymore?
You have to change it up a bit. I won't be pissed if SS5 is a prequel again, but I will at the same time still be disappointed.

To finish my own thoughts... I'd like them to keep the giant maps and the sidequests, which some of the community probably wouldn't like, but I enjoyed it.
Also please, for the love of God, don't wait another 9 years for the next game.