injector to spider fuel line size?

My new IO-360 Lycoming came with an-6 hose(3/8") from the injector to spider but the hose ends are -4(1/4")

I'm putting the red cube in this line and wondering if there's any reason not to use -4 hose with the -4 ends

No reason not to, and some reasons why you would want to.

The fuel in that line is heating up as it moves toward the spider, and the longer time it spends in that line the hotter it will get, which will cause issues on idling or low power operation when the engine is hot and everything FWF is heat soaked. A smaller diameter line holds less volume, meaning the fuel spends less time in residence (higher velocity) and picks up less heat.

Use a -4 hose.

Tom