Four is the four directions in the material plane...the square, a foundation, manifestation.
Six...ithe six directions ...now including the above and below...so a card of earthly (four) manifestion and of spiritual (above and below-2) growth...transitions from the past to the future...and eight being an more solid focus (two 4's, a time of stablity, one which can be alluring but which still calls for advancement in order to reach the ten.
I see the Six d'Epees as a time when one realizes that adaptations must be made for further growth, it is more "youthful" a card to me than is the 8....decisions are still being made here, and depending upon the placement and the topic, could suggest that one must be careful about choices made in this stage, since it represents an earlier phase of development that an eight.
Positive, young in spirit, perhaps a time of artistic conception...but definitely one signaling movement from the past to the future...resolving old ways of thinking in order to move forward.
terri