I recommend you enjoy an iPhone 4S if you can, but an iPhone 4 is an amazing device the same way.
you should make sure that the iPhone works with the carrier of your choice (rogers, etc), the sim card for iphone 4 is smaller.
would be cool to make a call before purchase too.
A used iPhone 4 should be ok, unless water damage.....play around with it before purchase, once it is working, you should be fine, just check for dead pixels or anything else your eyes can see, try turning on and off the phone.... an iPhone will never brick(guaranteed).
its very important for a buyer of a
used iPhone to check the
modem firmware on an iPhone, to make sure it doesn't say
06.15.00.
settings/general/about, thats all..... don't worry about the version, once it doesn't say 06.15.00, buy it.
06.15.00 modem firmware was taken from an iPad 3G to make an unlock, so that the iPhone can work with any carrier(jailbreak was required), but only a few people had issues with it dropping wifi signals and gps. Mostly found on iPhone 3G and 3GS, but nowadays iPhones usually come unlocked, so not much of an issue.
A quick ask of, was this phone jailbroken, to have an idea, but still check modem firmware.
Even if it was jailbroken, and is back to stock, phone is 100% OK to purchase(my word).
i Prefer a Jailbroken iDevice over stock, anyday!!!