Is that the limit? Because it sounds like a lot, it's like hitting someone with a tennis ball every second. (unless of course I'm misunderstanding the definition)
Cellphone don't put out 4 watts let alone 4 watts per kilogram of a human full human, they might hit 4 watts per kilogram if you are only looking at a small amount of flesh near the phone which is what SAR looks at.
Its like determining CPU performance by looking at perf per watt just because has a high perf per watt does not mean its fast or faster than a much larger CPU.
For instance dancing is around 5 wh/kg for your whole body more than actually getting hit with a tennis ball every second!
The reason it is relatively easy to surpass the limit in a phone is that you use it from quite close, and the energy density decreases exponentially with distance.
So something like 300W for a human body seems like quite a bit of energy. But maybe it's not like poison where it's maximum amount per kilogram bodyweight, but it's more like the maximum for any particular kilogram of tissue.
But even when pelting someone with uniform light rather than physical objects, a 300W IR lamp would be pretty noticeably warm I think.
Anyway it was more than I thought. I was expecting some figure that could easily be dismissed as 'impossible to cause any physical effect, let alone harm'.
Maybe that's why
Watts are per second, Joules are total energy (watts * seconds). 4 Watts would be per-second.