Am I likely to get a performance improvement upgrading iphone 4 from ios6 to 7.1?
7.1 is probably the best performance that you'll see on the iPhone 4. While the loading times for apps seems to be cut by a half a second or more for most, there's only so much you can do for a 4-year-old processor. I'd say the upgrade is well worth it. You won't see all the features that the 4S and 5 iterations see, but its still an increase in performance -- I'd say go for it.