As of HQ18, we've made a change to 4-Star Combiners, and this change which will effect all players (regardless of HQ).
3-Star Combiners can now be upgraded to level 20 and 4-Star Combiners can now be upgraded to level 25. We have also reduced the time it takes to upgrade combiners.
We have increased the health of all combiners by 25%. This is done to address the concern that combiners die too quickly on attack - Since we believe they are strong on defence, we've reduced the amount of health they get from the combiner outpost to compensate for the 25% increase.
Visit our Combiners page to see which Combiners have been adjusted!