Ask that they narrow the definition of competition, or that they delete that clause entirely, or that they pay you your full wages for those four years to compensate you for the loss of income.
If they're unwilling to budge, walk away. It's not in your interest to work for an unreasonable employer.
That non-compete is overly vague and ambiguous. Does competition of a Contract Research Organization mean most IT consulting firms? Does competition include all the client law firms that might need/compete with your startup's services? Those are huge swaths of an industry that you could potentially specialize in after doing the startup, but that you would be barred from. Furthermore, imagine if your startup gets purchased, or if the startup pivots, their definition of competition could even morph as time went by.
In Germany, that clause is unenforceable, but even if that's the case why would you create that kind of trouble for yourself.
Even if it's unenforceable, once you leave that startup, you want to leave on good terms and you don't want them to go and try to sue you or sue your future employer (even on a bogus claim). The same goes if you want them to provide a good reference. That's why, you need to nip that clause in the bud right now, or else, be willing to walk away.
Ask them to compensate you for your loss of income and loss of opportunity cost during that non-compete duration. Ask them to reduce the time period. And ask them to be explicit about the companies they don't want you to work for.
What are you supposed to do during that time? Forget everything that you've learned that's industry-specific and become a cab/Uber driver for four years? And after a four-year gap in your resume in your chosen industry, who would hire you anyway? Most IT companies/law firms/CROs wouldn't. Or if they would, they'd make you re-start all the way down near the bottom of the hierarchy.
Now if you don't even want to argue the point, strike that clause out, initial the change, and return the contract back to them so they can initial the change too.
Whether these clauses are normal or not in a company?
Yes, these days, more and more companies are introducing non-competes even if they have no business doing so. That doesn't make it alright.
Many businesses just insert that clause to see what they can get away with.
And please, don't fall for the "Trust me, we're not going to enforce it." Even if that person thinks he's telling you the truth, you'll be out of luck if that person gets hit by a bus or if the startup gets purchased by a bigger company. In such a case, the proper response is to say "Great! Since you're not going to enforce it, let's go ahead and strike that clause out right now."
If it is patent related then that makes sense.
No, it doesn't. A patent protects them already. Besides, a patent becomes public information anyway when it's published by the patent office. So a patent doesn't get protection through secrecy, on the contrary.
Now, it may be that they want to protect trade secrets not covered by the patent. But let's be honest here, a Contract Research Organization is not some R&D lab, also as a CRO, most of what they do will be easily discoverable through their marketing materials or by their clients.