Adding a seperate functions file for CPT declerations

Hello, quick question... i want to create file in which i will be placing my Custom Post Type codes (i have about 4 custom post types and therefore they really eat up the functions.php space)

Before anyone mentions it, i know code for CPT should be added to a plugin instead, however in this case i dont want to do that.

I added all my CPT codes into a php file and called it customPTs.php - i removed the code from my functions file, however the CPTs stopped working. So how do i call in this file? For JS and CSS usually you enqueue the files. What about for PHP files?