A 6~4 reducer with an AN6 female 37° flare seat on one side and an AN4 male flare on the other, so it steps a larger line or component port down to an AN4 connection without a stack of adapters. Typical uses include picking up a small gauge, sender or turbo oil feed line from an AN6 port, or terminating an AN6 hose end into an AN4 fitting. Keep in mind that the reduction becomes the narrowest point in that section, so use it where flow is genuinely low — instrumentation, bleed and pilot lines — rather than in the middle of a main fuel or oil supply run.