How should I cope when a spouse has stage 4 colon cancer?

Find resources! Make sure you have the information you need--and realize it might be different than what your spouse needs. Make sure you take care of yourself and have your own supports--it's a lot of work to be the spouse of someone with stage 4 colon cancer and you need to care for yourself to do the best job for your spouse!
It is difficult. If you have a pastor, that would be a good place to start. Also the american cancer society has many resources available to you and your spouse. Consider asking your doctors for a consult with a palliative care specialist. Hospice is also a godd resource for patients and their families. Be supportive of the spouse and get help from any or all of the above resources.