I think some people are going a bit overboard with their recommendations - you're going for a bike ride, not trekking in the Himalayas.
Bear in mind that the more stuff you carry, the greater the weight, and thus the more energy you'll have to use to shift it (especially uphill), so be prepared but try not to overdo it. You'll be riding for 3-4 hours, it's not a major expedition.
One spare inner tube should be plenty, but take two if you want to err on the safe side. Plus tyre levers and a pump. And practise using them before you set off to make sure you know what you're doing.
For a ride of that length, one large water bottle or two small ones should suffice - unless you're riding in a really remote area, you'll have opportunities to refill, eg at corner shops or cafés. That's a better policy than carrying lots of water with you (water is very heavy).
Some food will be a good idea, but again don't overdo it - a banana or two, maybe an apple, a bag of jelly babies. Plus money for a cup of coffee and piece of cake at the café stop. And eat a hearty breakfast an hour before you set off (give it time to digest).
As long as you check your bike thoroughly before you set off, mechanical problems shouldn't be a major concern on a ride of that length. Take a multi-tool in case you need to make minor adjustments on the road, and a mobile phone to call home for rescue if anything more major goes wrong. It's not worth trying to carry out major repairs at the roadside unless you really, really need to finish the ride.
Clothes-wise, check the weather forecast and dress appropriately. If it's dry when you set off, take a lightweight rain jacket just in case.
Most of all, enjoy the ride. If you're not used to rides of that length, go slightly slower than your usual pace to make sure you conserve some energy. If you're still feeling strong towards the end, you can increase the pace for a fast finish.
d.