5S batteries down to 4S batteries

[JLO]

I have 6 Thunderpower 6S 6600 mha 25c Batteries brand new that I bought under recommendation from the motor dealer but now it turns out I need 4S batteries other than buying the 4S batteries and put the 5S aside for a wile, is there a product that I could use to down grade the voltage to mimic 4S batteries or is this a bad idea.
thanks

 

[redridinghood]

FYI

http://www.dream-models.com/eco/LipoPack.html

 

[JLO]

Well I was hoping some kind of down converter that could handle the load and bring the 18.5v down to 14.8v