I too would like to know if it is possible to use 2x4's as my support boards to lower the height. I will be using composite wood as my surface boards.

Advice?

From the FAQ that explained this (I was in the FAQ looking for something else)

We do not recommend building an entire deck using 2"x4" support boards. 2"x4" supports can be used in some situations where height is critical, but only for less than 20% of the entire support structure. For example: If the entire deck has 10 support boards, no more than two should be 2"x4"'s. If 2"x4" lumber is needed, position a Dek-Block® pier every 30" along the support board.

Hope that helps you.

We do not recommend using 2x4 as joists. Please call the technical support line so this can be explained further.

Deckman

Alternative: bury the dek blocks 2" each still using 2x6 supports to accomplish same desired height as 2x4s would achieve.