No, that's not what I'm saying. With or without Alaska, your B2 visas are valid for some extended time (when I lived in the UK, my B1/B2 visas stamped in my passports used to say "valid for multiple entry" and were good for 10 years indefinitely).
Visas do not actually grant entry to the US; They only allow arrival at a port of entry. Entry beyond there is granted for some duration (6 months max) by an officer/agent, and the "leave by" date will be recorded on form I-94. The I-94 is/used to be the landing card that you are given on the plane before arriving in the U.S., and stapled in your passport by the officer. Nowadays, I believe they use electronic I-94's, so they may not staple a paper version in your passport, although the "official" web sites still refer to the paper form.
My simple reading of, and answer to, your original question is:
1. If/when you re-enter the U.S., there should be no reason you wouldn't be allowed in, and you'd probably be given a "fresh" 6 months stay.
2. The unnkown (in my mind) is if they would issue you an I-94 on the cruise ship when you dock in Alaska. If they did, it would initiate a "fresh" 6 months entry.
3. Assuming the '4 months in Canada' is after the cruise, one way to not have the clock continue to tick might be to officially end the U.S. stay when you arrive back in Canada. If nobody collects the I-94 or records the end of stay, there are a couple of ways to notify the authorities.
4. Presumably, you'd wish to re-enter the U.S. after the remaining 4 months in Canada. You need to know if any of the above will affect your re-entry.
I need to refer you to some "official" links for you to do further reading and fully answer your question. But, I'll mention one concern should you attempt to enter the U.S. three times in succession; We've (anecdotally) heard of one British couple who were denied the third entry because it appeared to the officer that they entered Canada multiple times merely for the purpose of getting around the system and extending their total time in the U.S.