Which places in particular? Here in the middle east temperatures regularly get up to 45-50 degrees celsius in the summer and people get by okay. Most places have average temperatures much lower than that, so how would a rise of just 3-4 degress make then uninhabitable?
Another thing noted is that urban features such as green spaces, water features, and increased shade may work where exceedance is typically nonhumid (i.e., North America, Middle East, Australia), these factors may not be as effective or even actively detrimental in more humid environments due to the added humidity.
Largely because the 45-50 degree temperature do not last very long. And partly because AC loss due to prolonged blackouts has not happened during such heat - yet.
A 3-4 degree increase in average temperature means that 45-50 degree temperature periods may go from lasting 1-3 days to 2-3 weeks. And that's a whole new ball game.
Bangladesh alone is +170M people. Dispacing these people will be a refugee crisis on an unseen scale.
The issue isn't all thermometers going up by 3.5 degC. The issue is that it also brings a host of other changes, and it's those things that kill you. They include:
- higher variability of temperatures. So your +3.5 degC actually makes heatwaves of +10/+15 more likely
- changing rainfall patterns, taking rain away from some places that rely on it.
- rising sea level, where the small average temperature changes are sufficient for a significant increase.
I actually think the focus on global average temperature increase is a major failure of public science communication, precisely because everyone mentally adds the increase to their local climate and thinks "meh".