In the event that you ordered the wrong kit, these are the parts you would need to replace the 2040 Y axis of the Ender 3 Non Pro kits with the 4040 Y axis that the Ender 3 Pro uses.
1x 4040 x 550mm V-Slot Black extrusion with belt tensioner mounting holes drilled and tapped.
1x GT2-6mm Y axis belt for Ender 3 Pro 400/400XL kits