Do I need a SIM card to use my Iwatch series 4

The watch uses an electronic SIM that is build into the watches hardware. When you activate cellular the carrier sets up the electronic SIM.

You do not need a new line. You carrier assigns a phone number for the watch but they only use that for there accounting . It means nothing to you . You can't use it for anything. Your watch uses the same number as it paired phone.

Expanding on what lllaass said, the cellular version of Apple Watch must be configured by your network, as a second device on your existing iPhone calling plan. The watch has its own SIM permanently built into the electronics and they just need to enter its details into your plan. You keep your current number, and the watch and phone share the number so when a call arrives both devices ring together. The only reason you might need a new SIM would be some obscure technical issue like you have a really ancient SIM and they need to upgrade it, but you would simply be a replacement SIM for your phone and you would still keep your existing number.

Read https://techcrunch.com/2017/09/12/the-cellular-enabled-apple-watch-has-the-sim-built-in/

You will get another "phone number", however, that is only for your carrier's accounting purposes. The watch uses your phone's number for all functions.