Heat exchangers have a long parts warranty...either 10-years or 20-years or sometimes a lifetime warranty. Check your manual if you have it.

This covers the part only and not labor. Still, find out if part under warranty and how much for labor to fix. I don't see a reason to replace the entire unit unless your warranty covers the entire unit. Some companies offer replacent of entire unit to original owner, but not sure carrier or your model is one of them.

A cracked heat exchanger in a 4 year old furnace is very unusual. Did the tech show you the crack? Did you install this furnace?

The part will be covered by the Carrier warranty. You will have to pay for labor unless you have extended warranty coverage.

A cracked heat exchanger may have been caused by a bad installation. The furnace may have been operating too hot. You don't want to repeat this again. I would ask the tech to check this regardless of which option you take.

If you do get a new furnace, ask if Carrier is willing to make a good faith gesture by offsetting the cost of a new furnace. It doesn't hurt to ask.

Mike is right that you need to verify heat exchanger is cracked.

Also, I think there's a risk of bad fumes.leaking through cracked heat exchanger, so its dangerous to use since you could die from these fumes. At least that's what an installer told me.

The part is under warranty. There was a class action suit against Carrier because they switched to an inferior material which did crack in many instances. We're concerned that the replacement part might also be a problem in the future. This repair sounds like a major one. We built our house 4 years ago and suspected that the installation might not have been the best. Also had problems with the water heater not being installed right.
We're expecting to replace that soon also. The furnace stopped putting out heat, but the blower ran constantly when we turned it on last week. The technician cleaned and checked it. Thought it was okay and was ready to leave when it cut off again. The furnace was very hot to the touch. He determined that there is a crack in the heat exchanger. We're waiting to hear from him after he contacts the manufacturer.

"The furnace was very hot to the touch"

This is a bad sign and the likely cause of the heat exchanger cracking. It sounds like the high limit switch is shutting down the furnace.

Carrier did have heat exchanger problems in the past. However I would have thought your furnace would have the improved version. In either case it should be under warranty.

We got another company to look at the furnace today. This one says the problem is that the venting pipes are not sized correctly. There is a 2 inch pipe coming out of the furnace which is connected to a 3 inch pipe. The pipe runs for about 12 feet and out the basement wall. The tech said the whole pipe should be just 2 inches because the run is less than 15 feet. He says that we have a blockage from soot which is causing the furnace to trip off due to excessive heat buildup. He also determined that the heat exchanger is not cracked. We have no warranty to use toward a new furnace due to bad installation. Both men recommended a new furnace.