How much storage will be taken from me if I updated to ios8?

So a new update came saying that improvements are made for those iPhone 4s who updated to ios8. I'm just wondering if the storage taken will be bigger than the update on ios7.

My phone was 16gb but when I updated to ios7 it became 13.3.

I have now around 3.5gb -4gb will that be enough?

If you're on iOS7 don't update to iOS8, it will slow everything down and your battery will die a lot faster. The most recent update helps with that a bit but it isn't as good as just keeping iOS7