Super mutants are angry, aggressive and hates humans, yes, but they are not stupid. First and foremost they utilize tactics; they take cover, use grenades and even help each other in battle. Second, they know how to operate advanced weaponry like minguns and even rocket launchers, and how to construct and maintain armor. They also seem to be able to know how to program turrets, IIRC. They fortify their camps in strategically sound locations; cross-roads, elevated positions, construction sites that offer multiple vantage points, etc. Furthermore, Rex Goodman was given a radio to call for help, explicitly for the purpose of luring other humans into a trap (which they also did at another location, can’t remember where though). They also show ability to train and command animals with their mutant hounds.
What makes muties in F4 different from previous games (Fallout 3 notwithstanding) is their unparalleled hatred for humans. However, considering it was humans who made them what they are (the Institute), they have a pretty good reason for hating what they once was. It could also be argued that super mutants genuinely believe they could rebuild the world, as unlike raiders there have never been two hostile groups of super mutants fighting. They all cooperate. Strong even says they operate on a rudimentary system of communism, where they share all their resources equally.
In short, Super Mutants in Fallout 4 are not stupid. They are just very, very angry.
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