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IMO, #3 was more about YAGNI[1], i.e. you probably won't need the speed, integrity constraints, relations, etc. delivered by SQL, so keep your data model flexible and lightweight until a shortage of those things is actually getting in your way.

I wouldn't be surprised if many people criticizing #3 had no practical experience with NoSQL and the benefits brought by its lack of baggage.

[1]: http://en.wikipedia.org/wiki/You_aint_gonna_need_it

"The order of examples is not necessarily bad first, good second." http://news.ycombinator.com/item?id=2203101

SQLAlchemy is "complex" in both #3 and #4, so it wins in #4.