Discussion about Sylvan (Wood Elves).

The choice depends on many factors. I usually start building upgrades after all the dwellings are available in my town. In principle, if the main hero or the second hero is Ossir, plus there's a dwelling on the map nearby, and the map is rich in resources, you can try upgrading and then run off to find wood. :)
On a small map, I wouldn't sacrifice 4 dwellings – you might simply not have enough of these tanks in a battle.