Blue Anodized Aluminum Adapter (AN 919), -04 Male JIC 37 deg Flare To -03 Male JIC 37 deg Flare, Straight

That's what this adapter does. It's a straight fitting with a -04 male JIC 37-degree flare on one end and a -03 male JIC 37-degree flare on the other. Both ends use the same 37-degree flare seat, so as long as your mating fittings match those thread and flare specs, it'll connect right up without any issues.

For stepping between those two JIC flare sizes, this adapter is set up exactly for that job. It is a straight -04 male JIC 37 degree flare on one side and a -03 male JIC 37 degree flare on the other, so it lets you connect a -04 female JIC end to a -03 female JIC end.