Well you will get some dip over 3-4 years of daily charging. Rechargeable batteries gradually lose capacity with use - normally their lifetime is quoted in terms of the number of complete charge cycles before the capacity is reduced to 80% of the original, which depending on the battery can be between a few hundred cycles to over 1000 (with a partial discharge and charge counting as part of a cycle). I have no idea what the official rating of the S6 Edge's battery is, but if you use a large fraction of the charge each day you should obviously expect some reduction over 3-4 years. Of course over that time varying usage and software updates (which can improve or degrade battery life) will also have an effect.
What I would say with a fixed battery is don't abuse it. Don't run it down to flat on a regular basis - that will degrade the capacity. Also heat is not a battery's friend, so I don't recommend regularly gaming while on charge, for example.
My M7 has a fixed battery, is 2 years old and is still fine. But I cannot promise you that you'll be good for 4 years - it will depend on your usage and requirements. If it gives you a lot of margin now you may well be fine, but if it's just good enough at present then it won't be in 4 years.
