There’s normally no combat during the start of Fallout 4 but IGN messed around with the game to introduce 50 synths to Sanctuary Hills before the bombs fell. There’s also a behemoth eventually.
Interestingly, the residents of the suburban area don’t takes things lying down and fight back, with some even picking up weapons dropped by the synths.
A lot of the residents seem to be carrying ammo for handguns so looks like some of them were prepared for robotic visitors from the future.
Maybe this is how Bethesda should start Fallout 5, you can watch the video below.