I'm back to ST4X and really enjoying The Rim 64x64 (42 worlds) map on Hard. (As and aside, I find the end-game on larger maps requires too many individual moves per turn, this this medium size map is feeling like a good size).
Early in this game, Zenga systems were discovered 11 tiles from my primes, with an unsettled Red Dwarf a few tiles closer. We made it a priority to keep the Zenga off the nearby Red Dwarf, to prevent them from using it as a bridge to invasion. And, also, to preserve it as a jumping off point in our eventual invasion of their space.
We used Attrition Warfare to keep them off the dwarf, harrass them and bleed their strength. This consisted of periodic patrols into Xeno space to locate and destroy Zenga vessels, retreating before our ships sustain much damage. At first, only one Templar Defender was needed. The most recent patrol, turn 95, included both Templar Defenders and a Chaff Fighter and destroyed two Zenga Colony ship and several other vessels of war. The success of this approach is further supported by the relative weakness of the Zenga--level 7, 3 worlds--compared to the Krang--level 7, 9 worlds--with whom we've had no warfare.
Attrition Warfare is related to other tactics like Reconnaissance in Force and Defeat in Detail (which were discussed in a previous thread).
The Chaff fighter is an inexpensive fighter with high evasion and guns, and low hull and repair. The name and concept was developed by another board member, who set repair at 0 and viewed them as disposable--my version is less radical, with repair of 3-4 and durability of 4-6.