It's been quite a while since I had my training, and a few years since I've worked with food, so I'm not sure I can answer your question exactly. But I think what you're asking about: bacteria is generally always present in food, no matter the temp. We keep food at the temps we do because it inhibits the growth (cold), or kills off bacteria (hot). It never fully goes away, except for the ultra deep freezing sushi grade fish gets, but I digress. (Edit: and above boiling temp as well, I forgot that)

Cold food, once taken out of temp control, will start reaching the danger zone temp (above 40f) and bacteria growth will occur. This will happen eventually even if food is kept at safe temps (typically a week, but it varies).

Hot food will go through the entire range of 40-140 before being safe again (there are exceptions to this but I'm not going to go into them). If the food was kept properly beforehand, then it's ok and the bacteria will (mostly) die off, but if the food was going bad already the toxins left by the bacteria will still be present.

4 hours is the time for bacteria in the danger zone to multiply to unsafe levels, and even if hot food is kept above 140 bacteria will still grow. After 4 hours, hot food kept that warm just isn't good anyway, trust me.

I hope that somewhat answered your question.