I won't write a super long review on this, as you most likely already have seen the first 3 seasons when checking out reviews here.
Season 4 is in my opinion the best of them all. The story is well done, the relationship between Louise and Saito breaks its partially repetitive pattern and finally advances, and with some major tragedy in between comes to a truly beautiful happy end.
The ending is so well-rounded that I'd say that is the optimal ending the series could've had, no need for more seasons whatsoever.
**SPOILERS AHEAD**
If you want to be spoiled what happens in their relationship (very shortly); Saito gets another power which allows him to increase void magic (e.g. of Louise) at the cost of his own life energy. Louise confesses her love to him right before they part ways (she tricked him so he wouldn't risk his own life), but Saito manages to get back and after winning, proposes to Louise. They get married, and Saito shows her his world.
