Does Apple Watch Series 4 need an iPhone?

iPhone is required to set your Apple Watch, of course and to use your watch in general. For example, you can't update your Watch without an iPhone. Apple Watch is not a stand-alone device, it's a continuation of your iPhone if you wish.

But you don't have to bring an iPhone with you all the time. There are two models on the market: LTE-version and GPS only. So that leads us to what can you do with the Watch without your iPhone nearby:

How to use your Apple Watch without your iPhone nearby - Apple Support

The iPhone is required for setup of the watch. If using a non Cellular model, the iPhone is also required to make and receive calls on the watch, and send and receive messages on the watch.

While the watch can be used without an iPhone nearby, it does for those features require it to be connected to Wifi, and the iPhone needs to be turned on and connected to either Wifi or cellular data for messages and calls to get to the watch.

A GPS+Cellular watch, does not require Wifi, but will need to a good LTE signal to receive calls and messages.

The watch is in general an extension of the iPhone, it uses the iPhone's number and service plan for calls and messages. The cellular model adds an extra cost to the paired iPhone's cell service bill.

The watch is not a standalone device.

See here for more: How to use your Apple Watch without your iPhone nearby - Apple Support

iPhone. iPhone. iPhone.

Yes, they do.

Apple Watch Series 4 require an iPhone 6 or later.

iMac can't setup and update the watch.

Apple Watch requires an iPhone. Period. There's no way to bypass it.