After a bit more than 400 hours, I suppose I can finally feel confident I know enough to review this game. I'll focus on the questions you might have when you see a free fishing game and the tags it has, or at least the questions I had when I first saw it. So without any more delay, here's my thoughts on Russian Fishing 4.
The big one- "Free". How free is it? I have not paid for over 400 hours now and I've enjoyed my time fishing. So I would say it is very viable to play for free. The price for premium is roughly on par with a normal MMO's subscription at 12 euro/month. You get a few perks out of it, but mostly it's just extra exp so you get to level up faster and get to unlock other areas and skills easier.
As for the multiplayer aspect, I find this one just straight up weird. Only a few parts of this game appear to have any multiplayer to it. Aside from those parts it is essentially a single player game. These parts are a leaderboard and chat function. That one explains itself and makes sense. A competition system, but one that I've never found a use for since all the contests are in Chinese or Russian so I have no idea what they're asking, and they seem to be limited to higher level players anyway so for about 300 hours I never qualified to get in even if I could read it. The ability to send items to other players only if you have paid for premium, and lastly the shop inventory can be sold out if other players buy a lot of it in a short time period.
You do not get to sit next to a friend when fishing, see others fishing, or even really see what other people. Your only interaction with others is through the chat window, and the occasional message in chat when someone catches a sizable fish.
The gameplay- It's fishing. You cast lines and reel in fish. You can fish with bait both by float fishing and bottom fishing, and with lures both from the shore and by trolling. The way you fish seems to have an effect on how well you catch things, while time, temperature and weather matter as well. The detail is better than any I've seen before, and it feels like you can actually control what you catch and whether you're successful by paying attention to the circumstances. The gameplay itself is good. You sell fish you've caught for money or can release them for bonus experience, but that's not really an option. You need the money.
The progression however is less good. I've spent a lot of time on it and I still use some of the beginner gear, and I've still not unlocked most of the areas. You never get enough money to buy anything, and I know you're thinking they want you to buy the premium currency for it to skip the grind, but they don't really support that. If you buy premium currency to get gear you will need to spend around the price of an actual physical fishing rod to get early to mid level gear. And that gear will deteriorate very quickly, requiring that you pay even more to get it repaired. It is honestly cheaper to just buy a fishing rod and go do actual fishing than it is to try to spend money on gear in this game.
Your progression is limited mostly by money. You can unlock new places by leveling up, but what use do you have for a place known for fishing large carp if they'll destroy your gear with ease and then just swim away? Progression is very rough and there is no way to get around it. You will always be struggling for money and be limited by your gear.
In conclusion, while I find it very strange how they handled balancing just about anything I have enjoyed my time playing Russian Fishing 4 and would recommend giving it a try. If you enjoy the gameplay and just want to sit down and fish, you might enjoy it too. But don't expect to reel in huge fish, or even just somewhat big fish. You're going to be fishing in the same waters with your small rods for a very long time before you can unlock anything else.