If you are VERY concerned (to the point you can't sleep at night) then go with 4". If you LOVE the 5" so much that you are willing to "live" with the seasonal gaps that occur, then go with 5" in a quartersawn cut.

http://www.thehardwoodfloorcompanykc.com/shell.cfm?MainCat=41&SubCat=202

Excellent link explaining what is going on with wood. You have humidifier and dehumidifier already in place. You are several steps ahead of the game!

And to be clear, you've chosen a wood that is less stable than others. If the possibility of gapping is so intense that it is upsetting, then perhaps a different wood could get you want you are looking for without so much worry.

@rl2003and5 here's the skinny:
Hickory does have noticeable movement in service. If you do a glue assisted install with your (de) humidification system you should be fine. If you still have concern go with a hi quality engineered site finished hickory(4mm wear layer) and still do a glue assisted install. Owens Plank makes a great unfinished eng product. Do your homework and hired a certified (NWFA) installer. I've done a 4" glue assisted 4" in our climate with zero problems. I've also done a 3 1/4" solid with some problems in my area. Do your homework!! Good luck. Hickory is a pretty awesome floor!!

I just want to thank you two who have offered some great info and advice! I appreciate your input! I have decided on going with the 4" hickory. I am awaiting a quote on the cost to get engineered unfinished, but that may be cost prohibitive. My contractor said that they usually nail the 4" solid hickory and not glue and nail. What are your thoughts on that?