Using Apple Watch Series 4

The answer depends on what version of Watch OS you are using.

If it is a watch OS5 yes, if it is OS6 then the answer is no.

You should be able to, but why don't you update both your iPhone and Watch , unless you have an iPhone 6 or earlier. It would be helpful if you give us the complete picture.

MaximLtt wrote:

I'm thinking of buying Apple watch, but I can't afford to update my Iphone 6.

If I were you, I'd save the money and use it toward updating your phone rather than spending it on an Apple Watch.