I'd originally entered from Via Santa Vienta and then gone all the way down to the lakes in Santee. (It's not signed except to say "LIVE FIRE AREA" to the west side of that trail in the valley, so if they want people to say out, they need to put up better signs.) But to go over to to Building 4, you'd have to go through at least one Do Not Enter sign. I really get no thrill from trespassing and at this time of year the land will be pretty parched and hot. Plus urban explorers have kindly already posted the fruits of their risk-taking, and there's gross stuff in the ruins (cadmium, etc.) - here are some excellent photos, and below is one of the videos of the blast shield.
Possible exhaust flumes.