D&D will never be the number one priority for your friends. Jobs, relationships, school, other hobbies and many other things will always compete for their time.

I can't simply give up a block of time months from now to D&D.

So yes, 4 hours every other Saturday really is too much to ask. Any automatic time frame that arbitrarily puts D&D ahead of all the rest of my life is too much to ask.

I just received an email from my DM - he's checking to see if enough people can play this Saturday. We did last Saturday, for the first time in a couple of months.

That's normal life. D&D does not come first even for only 4 hours every other Saturday.

You'll have to schedule each game separately, just like you do every date, every shopping trip, every vacation, and every other occasional activity.