Hello I recommend you speak to your partner about this, voice how you feel and allow him to explain himself too because maybe there’s a reason behind it. It may not be that he’s not getting turned on, he may have experienced some trauma as a child or earlier in his life which has made him self reserved but I don’t want to assume the worst.
However out of all this there is something positive and that’s the fact you both love each other and the fact that despite everything you’ve still managed to stay together for 4 years and that’s something a lot of people wouldn’t be able to do because sexual intimacy is what brings partners and relationships together and many thrive and rely on just the sex not the psychological and emotional attachment they might have with their partner. So for being able to maintain a relationship for so long without sex it’s an achievement on its own and shows that there’s a much deeper love between you so don’t give up even if it’s been hard, all that’s required is a little bit of communication.
Good luck !