[XorNot]

To be very specific: people who think increase isn't dangerous need to look up web-bulb temperature [1].

There are regions of this planet where an average 4 degrees increase will mean that for periods longer then 24 hours, multiple times a year, the wet-bulb temperature will exceed 35 degrees C. That is unsurvivable by human life. Your only options in that environment are to not be in that environment by either escape or technological means.

There are cities built in areas where this is a risk, and if it happens they will just be depopulated: it is not possible for millions of people to escape an urban area under a heat wave. If they're somewhat built up, then they might survive it provided the electrical grid holds out - which, as recent experience globally should show - is questionable. Remember: under these conditions, no repairs to external infrastructure are going to be possible - you would need active refrigeration to move around outside and survive. We are not remotely adapted to that sort of hostile environment.

Most likely we are a decade or two out from a climate-forced mass casualty event, probably in the Indian subcontinent, with more minor (slightly less mass death) occurring in parts of Asia earlier.

[1] https://en.wikipedia.org/wiki/Wet-bulb_temperature