With four shammies, mine look something like this:

T - X - F - X
F - X - T - X
X - T - X - F
X - F - X - T

T = Thunderstorm
F = Frost Shock
X = no-cast " , "

Shammies 1-2 cast first. Shammies 3-4 cast second. Shammies 1-2 cast third. Shammiess 3-4 cast fourth.
This will allow faster back-to-back casts of Thundershock, without having to wait for the GCD.