Should I buy the Series 4 Apple Watch, or just wait for the Series 5?

I heard the Series 5 may be released around September 2019 but that may be wrong. What I really wanted to ask is this. Do I need to have cellular on my Apple Watch to be able to pair it with my iPhone? It's like $100 more and I just wanna be sure with that amount of money.

Given that Apple releases a new model every year since 2015, it is a surety that they will in September as well (re: Series 5)

No, the Apple Watch is paired via bluetooth. The cellular connection is more for use independent of the iPhone. The ones without cellular could only fully function if the iPhone is within bluetooth range.

If you are one to have your iPhone with you anyway may as well save the $100 & get the w/o cellular model.

As far as the Series 4 vs 5, its simply a matter of your patience.
Series 4, now
Series 5, in 6 months