Have you asked your boss about business insurance ? At 17 I had a 200 bhp clio and it cost £300 on business insurance.
Granted you don't build no claims but your age will bring your premiums down. At 19 I had a golf gti and it was about £600 to insure. I'm now 21 and almost every car is under £500 to insure and I only have 2 years ncb
I would not pay 4k on insurance though, if getting your own policy is only option then get something cheaper else you will regret it. Bare in mind if your paying monthly and lost your job you still have a 4k insurance bill to pay
A lot of the cost depends on other factors such as where you live, it won't be the same for everyone. I had to build more like 6 years NCB to get below £500!
Financial commitments are worth thinking about for sure - what would happen if you or your partner lost your job, could you cope if you suddenly needed to do repairs at home etc. Also don't forget that your financial commitments may affect your ability to take out a mortgage in the future.
I know what it's like to be trapped in to finance and suddenly find you can no way afford it (my other half had a new car and was unexpectedly made redundant in the worst possible way), and it is not bloody pleasant I can tell you. We're still recovering three years down the line. I would urge anyone to think extremely long and hard before making any sort of financial commitments, you really don't know what's around the corner.