When will a quad-core Intel CPU use its max quad core turbo?

if only a certain core is being stressed then that core will turbo and if its in a heavily demanding situation then all cores will turbo but it will only turbo when the cpu temp is below a certain level otherwise the cpu will overheat. This is why its a turbo because its only good for 10-20 mins heavy load then it will stop because the temperatures get to high(stock/laptop). Most desktop non-stock coolers keep the cpu cool enough to use the turbo indefinitely but laptops can almost never keep the turbo up for long

The simple logic as you mention is that if a core is used, then turbo kicks in. If 2 cores, then 2 cores are turboed.

If all 4 cores are used, then all 4 cores will use turbo.

Difference - when you have only 1 core on turbo, it can overclock to higher frequency than when all 4 cores need to be turboed.

For all four cores to be turboed to the frequency of 1 core, more voltage is needed than the one specified the CPU will go to usually. That is why when you see the CPU, max turbo on 1 core will be higher than max turbo on all 4 cores (unless you manually adjust voltage and clock frequency).

the cpu will use turbo if it needs more power... if it does not need more then it wont use it... it does not matter what core you are using or how many