Resmed VPAP IV ST versus VPAP ST

I've been using my trusty old Resmed VPAP IV ST for over 10 years now and I have grown very attached (but mostly, used to) to it. Recently I started to feel the pressure wasn't as strong as it normally was and I figured it was finally starting to conk out. After doing all the usual checks (filters, tubing, humidifier on/off, etc) I decided it was time to get a new one (unfortunately due to how old this model is, nobody is willing to service it here in the UK). I searched online and bought a replacement from the US, but when I looked at the machine today I realised I'd ordered a Resmed VPAP ST! The machines look identical but I'm wondering if anyone can tell me what the differences between them are? The first thing I noticed when I switched it on to test it was that the exhaust was a bit louder than my old one. Not sure what's up with that.

Rather annoyed at myself for not looking closer and checking to see what I was buying was the same model. Also, before anyone asks, I have been using the S9 which is my backup until now. I do not like it because the humidifier is rubbish even on maximum settings (with climatecontrol set to manual as well and using the heated tube). So I'd really like to go back to my S8 but now I'm worried this one won't be the same. :/

Have you contacted the seller to ask what happened?

It's not the seller's fault. It's my fault for not looking more closely. I did not know that there was both a IV ST and just a IV. In my excitement to get it I must have missed that small detail.

It's done now. I just wanted to ask if anyone knew what the difference between the two models was. All the settings etc are the same.