I had the droids steal tech early on in the game. I got to a point where I couldn't steal anymore tech, however because I know you can steal speeders but for some reason they weren't available yet the Empire was able to progress over to tech 4. I guess the devs should take a look at this.
The Empire has first to progress on techs so that Rebels can steal their tech. If you steal eveything but the Empire doesn't progress, then you wait until they do so.
Yeah, the Empire had up to tech 4 because they were using Tech 4 vehicles such as the anti-air speeder vehicle but I was still prevented from advancing. The devs need to fix this in their next patch.