is iPhone 4/4s HSPA compatible

No, still GSM or CDMA

Yes, they certainly are HSPA compatible as is the 3GS. While the 3GS only supports HSDPA, the 4 and 4S support HSPA+

3GS Specs

4 Specs

4S Specs

Only the iPhone 4s supports HSPA+, the iPhone 4 does not.

Not to be rude, but read the specs that Apple has posted for the 4 and 4S, see the link in my previous posting.

Both the 4 and 4S support HSUPA and HSDPA. Supporting both of those is what constitutes HSPA+, per my understanding of the technology.

The 4S is capable of achieving a faster download speed than the 3GS and 4 because it supports a higher level of HSPA.

I did read them. I have the iPhone 4. The 4s is capable of higher download speeds because it supports a higher level of HSPA. It is called HSPA+. T-Mobile calls that 4G and at one point AT&T was supposedly trying to get the iPhone 4s to display 4G instead of 3G when it was connected via HSPA+.