Multiple GFCI with two hot wires and shared neutral
A GFCI can only sense a difference in currents which flow though that GFCI.
In your example, the current for your toaster flows through GFCI #2, but completely bypasses GFCI #1 (and #3 &amp; #4).  

The other GFCIs are not 'aware' of this 5A toaster current, because current flowing in the neutral wire doesn't flow through these GFCIs.