As far as i know icon (3) appears when telephone and cable is laid. I gues that (4) may be radio contact.
What is the meaning of colors?
Forgive me mistakes in editing if any.
1) Unit can do somthing, but blocked (orange color = disable/warning) by "AI ctrl" option
2) Unit do somthing (cyan/blue color = process) - wire laying, indirect fire, etc ("AI ctrl" option unblocked)
3) Send order/info through wire
4) Send order/info through radio
5) Recv info/order from high commander
6) Recv info/order from subords (or cmd of the same level)
7) "..." Recv info through voice (or inner phone) channel
8) "." Recv info/order through voice (or inner phone) channel in squad/crew bounds
*5-6 may be swap