Its very simple they used the same pump head on the xp1,2,3,and 4 the only difference and why you see a difference in flow is the length and number of blades on the impeller. It is simply cheaper for them to use the same part without engineering 4 different pump heads, when they could use the same pump head, impeller shaft and magnet. The only difference is the impeller blade.
I assure you it works. Ive had an Xp4 impeller in my xp3 for about 6 months.