I've just finished Your Lie in April and the last song at the last part of the episode, not the duet or the performance, when kaori says I love you multiple times, a song is played in the background and when watching the dub the lyrics appear at the top of the screen, I can't for the life of me find that songs name, could anyone help? I like the song and am desperate to know the title and artist. Thanks
I cannot comment in previous answer (low reputation) but Conman is correct
Kirameki (Kousei & Kaori Performance Ver.) | Ver.) キラメキ ～公生とかをりの演奏 Ver.～
I haven't got the episode in front of me to confirm, but I'm pretty sure you're talking about "Kirameki (Kaori and Kousei version)", a piano/violin duet version of the first ending theme. Here is a YouTube version you can check against.