4 cell is fine. 5 cell would have been fine too. If you LHS said it was too high, then I presume you are using servos that can't take 6V. You don't tell us what is in the plane so I can't judge, but most servos can take 6V.

Thermals are around all year round. Just had an 87 minute thermal flight in January in NY.

Easy Glider is a great plane. Enjoy it. Once you get used to it, try moving the CG back buy moving the battery back a little at a time. The further back the better it will thermal, up to the point were it gets too hard to handle.

Also does great on the slope. I have the EGE and I slope and thermal it all the time, all year round.