Should I buy an iPhone 4s now?

I've found one for a good deal online but I'm worried that when Apple does a new iOS update, the iPhone 4s will be too old to get it and I won't be able to update and use older apps like Snapchat etc. Which is basically what I'm buying the phone for haha! Or should I just buy a ZTE Blade V and save some money? Advice please!

Added (1). So I've found the ZTE Blade V for roughly £130 and an iPhone 4S (seller refurbished) for about £190.

Iv got an iPhone 4S and with the iOS 7 update and it all works and them apps should work

I'm using my 4s right now! Its a great phone, besides, the iPhone 4 should be eliminated of the major update. The 4s would live all through iOS 8. It'll probably hit the brick wall at iOS 9

4S should be fine, plus they hold their value well so even if you sell it in a year or two you will get alot back for it.

I don't understand why people post these questions all the time and almost never include how much they're being asked to pay for their phones. Yes - if you're weighing how good a deal is, how much you're being asked to pay matters.

Currently Snapchat requires only iOS 5, which means I can run it on my 3rd Gen iPod Touch (from 2009). Nobody can tell you that a new version will be released w/I 2 years that will require more than iOS 7, or whether the 4S won't be supported by that new OS version.

"should I just buy a ZTE Blade V and save some money?"

Are you sure you will save money? Your question doesn't lay out the price. Also, the Blade is an Android phone. Your preference for that phone or the iPhone 4S depends heavily on your preference for one OS over another. Is your current phone an Android or iPhone? If you've got one or the other, and you've spent anything on apps, you should probably stick with what you have. With Android you get greater control over your apps and their media, more freedom connecting to computers and the availability of removable memory (although the reliability of microSD cards in Android phones has been an issue). WIth iPhone you get Apple quality (my iPod is 3+ yrs old) and the assurance that the phone, its OS and the software that connects it to your computer are all made by the same company.