The language in the constitution is ambiguous, specifically thanks to the use of the word "unreasonable" ... which is way, way unfortunate, but (obviously, present circumstances evidencing) apparently more than enough equivocation to let checkpoints pass constitutional muster.
Then again, I'm not myself a huge fan of the constitution. It isn't terrible, but it's really not a particularly well-done document, specifically because of allowances like this (and others, of course... welfare clause, etc etc).
Speaking to the matter of the "reasonableness" of checkpoints, in any case, it has been argued that drunk driving is bad news, and it's been argued that these checkpoints occasionally catch someone in the act. It's also of course been established that driving is ultimately a privilege, not a right, and that these checkpoints occur on public property... the use of which is non-compulsory... which means that even if you or I as an individual may not like the statutes as they happen to be written, we have very little say in their administration.
You have said that you don't see how one can argue that checkpoints aren't a violation of the fourth amendment. I can see several sides of it, specifically because the ambiguity found in the language. I'd be all for shoring up the amendment so as to clarify just what "unreasonable" means. (Using case law to sort it out as time progresses is problematic for a variety of reasons.) But until that's been accomplished, it really doesn't make a whole lot of sense to me to argue the unconstitutionality of checkpoints... you can build a variety of houses on one kind of foundation, but if the foundation's flimsy, doesn't matter what you build, it's coming right back down.
I have no problem with laws against drunk driving, and I certainly don't drink and drive myself. It's absolutely no imposition on me to have a fifteen-second conversation with an officer about it. I saw the lights in the distance, I knew what I was getting into.