Personally, I'd only be running it on an iPhone 6 and above. My friend recently upgraded on her 5s, needless to say, it does run fine. Although, it has since become a little sluggish as compared to pre-update ios. I'd steer clear from updating the 4 to iOS 9.
Sluggish is fine. I'm only using it to test software I make. Other than loading an application from Xcode IDE onto the phone, it won't be used for anything else.
Its an iPhone 4s my bad. I have iOS8 on my current one.
Considering when ios 8 came out and a couple friends downloaded it on 4 and 4s, it basically slowed the phone down pretty bad, almost to the point where they don't even want to use it. Wouldn't advise it, but it might be alright for your purpose.

