Need to make an mp4 smaller without wrecking the quality

one caveat on H.265, older TVs, some work laptops and a lot of editing apps don't play it smoothly. if the file has to work everywhere, use H.264 at RF 20-22 instead. bit bigger, but it'll open on anything.

HandBrake, free. Pick the Fast 1080p30 preset, video codec H.265, constant quality around RF 22-24 and leave the audio at AAC 160. If the source is 4K, dropping it to 1080p is where most of the saving comes from and you won't notice on a phone or laptop.