EXPECTED: At 4 stacks, each different elemental attack extends the duration by 2 seconds, up to a maximum of 8 seconds.
ACTUAL: This logic works only for damage stacks, not for defence stacks. The defence stack logic was not changed during the Tal Rasha changes time some years ago!
How to reproduce.
You may use any hero like following:
Collect all 4 stacks by using 4 different elements and see the Tal Rasha’s icon with number 4 in panel.
Keep this 4 stacks by using the only 2 elements (teleport and swords attack as example routed to the mouse wheel for convenience).
Do it 10 seconds more.
Open the panel and see the number defence.
Press all 4 elements and see the defence is up.
This mean, defence need 1-2-3-4, but damage only 1-2.