I have been working in sales for my company for the last 4 years and year on year increasing my sales I have never signed or even seen a contract of employment.
They now what me to sign one that will restrict me going to a competitor for a number of months or speaking to my clients should I want to leave
Do I have to sign this? What can they do if I don’t sign?

They can let you go if you won't sign. There are no 'rights' to the job except civil rights regarding race, creed, religion, gender and age.
Some people hire a lawyer when this happens.
I might be inclined to ask for a discussion about re-writing it. I think it's reasonable not to talk to clients if you leave, but what if you have to relocate to take care of an aging parent or something (present a hypothetical that describes your situation)? I would not sign a contract that prevents me from working for a competitor. I might negotiate a small list of competitors, or a shorter time limit, if they won't budge on dropping that part altogether.