A6-3400m

it depend on the laptop, read the manual or go to the manufacturer website.
 
I don't know, you don't give us anything, like the make and model of the laptop.
 
Max memory is total max.

The total does not depend on the CPU (or APU). It depends on the way the BIOS has been designed. If the BIOS does not allow the laptop to recognize more than 4GB of RAM, then your total max is 4GB.

The last resort is to call Toshiba customer service and ask them what is the total max RAM recognized by the laptop.