Not plus 4 but it works… - Pro Mag Springfield XD-9 Plus 4 / XD-40 Plus 3 Magazine Extension

For restricted states(CA) you can add it to oem 10 rd mag and it will NOT increase capacity it’s just for looks but it does look badass, I tried this on a 16 rd oem mag and no matter how hard is pushed and even used a loader I was only able to get plus 2 for a total of 18, some say a adtermarket follower helps but it should mention that before you buy, looks great and badass either way, I have yet to try it at the range and will update when I do