The Four of Wands marks the completion of a phase. Often a card that denotes satisfaction, celebration, achievements, the Four of Wands is also a pause for rest in order to enjoy or assess what we have achieved so far. The Six of Wands, called "Victory", bears the element of interaction. It describes a situation that cannot exist without the contribution of (at least) two people and which usually results in a "win-win" outcome. The combination of the Four of Wands and the 6 of Wands shows the successful completion of a matter.
The elements of the cards are Fire (Four of Wands) and Fire (Six of Wands). They support nicely each other, that is why the stability suggested by the Four of Wands could become even stronger with consistent achievements and further success as predicted by the Six of Wands. If however, there are indications of conflicts and contrasts by other cards within the same spread, the blend of these two could lead to excess, arrogance and despotism.