Theirs no conuter that gonna get you a win without heavy loses (short of having all your units under a sheild) But if I remeber greande troopers and mech destroyers work the best grenade troopers are hellish if they get their spalsh damage upgadre.
Also, having an air force would do god unless the payer is smart to have anti air support.
Or u could simply use the THERE'S NO MOON cheta 5 times. That will be enogh.;) :D
Like Gaming Nut said grenade troopers work very well, and they are cheap. It would work best with a shield, but you will still have heavy losses.
Think combo. Put all of the above stated together in an attack group and you'll counter those pesky DD. You will take losses.
Just remember to push forward after eliminating them and get rid of those fortresses asap. (think massed bombers)
KL
I love using mass shielded droideka, especially before they received a penalty against attacking buildings (they are still good in mass, just takes a bit longer). Whenever I mass them, I also bring along 5-10 AA artillery and a collection of fighters to wipe out grenaders. With the X-Pack, Air Cruisers should also work quite well against droidekas, just make sure you have air support.
Kryllith
I agree with the combo counter to DDs....losses are inevitable but never be afraid of losing units if it effectively counters the threat, so long as you maintain the ability to crank out new units....
Little things can help turn the tide in your favor as well.....at T4 hopefully you have a ton of workers, 100+...It depends on how experienced your opponent is...once the DDs come near your base I have sent in like 20 workers to attack the dd force at the same time as my combo force....the DDs will try to hit the workers attacking them and those few valuable moments can buy the combo force time to do its job and give it an edge....now obviously this doesn't work all the time, its just that sometimes being creative and doing the unexpected things can buy you time or throw your opponent off....and those DDs get expensive to replace