I know that we have to draw FBD's for each block and then figure out the equations of each Fnet. But first, how do you know the direction that this system will move? Because for example, if you know that block A is moving down, we can assume down is positive and its equation will be

ma=mg-2T (because we denote down as positive)? Also, for block B, will the tension be 4T, since it's connected to four strings?

I am confused about how system works.

Thanks!