Yes, Forte was a Threshold company. Threshold was able to market products at a lower price point using the Forte name. I had heard the Forte 4A driving Thiel 1.5 and Martin Logan Aerius, and it was a great amp at that time for that price. Because of the power requirement of my Thiel 2.2s then, I ended up getting the Forte 6 (the 200 watt A/B version of the otherwise-the-same-design of 4A).
They share the same sound characteristics with their more expensive Threshold "cousins". Since you have both amps, why not trust your ears and make the judgment yourself?
FrankC