The difference between 4mm and 3mm would be a few pounds of weight.  The two heaviest components are the wood and the epoxy.  I'm finishing up a kaholo 14 right now where I've used 4mm luan.  It is turning out just fine although on the heavy side and the look of the wood grain is amazing.  Looks like it will finish up just over 42lbs which to me is a disappointment.  If I could do it all over again (and probably will)  I would order the clc kit.  Saves a lot of time cutting and scarfing as well as supplying the 3mm okoume which is ideal.  My hope is to build another one coming in between 30-35lbs.

Good luck, it's a fun build.

Thanks - actually found a good supplier of Okoume in Tarpon Springs, FL.  He's got 3mm and up for decent prices:  http://www.bedardyachtdesign.com