Yes, I like 4-6 hours. This way you can actually play a couple of days in one session, which IMO is important. Also, the duration of day and night has an effect on the rest of the time flow (and therefore travel distances and crafting time) unless you define that it\'s not our 24 hour day.
However, the season thing IMO isn\'t plausible given the setting (which you have read, haven\'t you?)... The only seasonal difference would be the duration of daylight cycles, no climatic changes. I consider Yliakum to be eternal summer. The lower levels might be cooler, but most likely not so much due to air circulation. They are darker, however.