Argh, an error ate my response. How frustrating! :P Let me start over but I have less time now.
I have to admit, you make a very good point sir. I like your inventiveness with the class abilities. I do disagree though with how powerful you think a druid can stop an army entirely however.
For starters, if the 4th level druid is trying to stop a small mercenary warband or a small tribe of goblinoids from invading then he could do it. They wouldn't have the resources to stop the druid. But a well seasoned army marching under one of the banners of the major nations of Golarion? They'd have ways to counter a druid.
If Entangle stops the first wagon in a supply train the first wagon will be stopped for four minutes (if a 4th level druid.) Assuming that the men don't break out axes to chop away the vines they just have to wait out the four minutes before moving on again. If the druid took Entangle two more times he could stop the wagon for 12 minutes total. Warp wood is 2nd level and could potentially ruin two wagon wheels since wagon wheels are medium sized. The army swaps out their spares, puts the other two in the back to be fixed when they rest for the night, and then moves on throughout the day.
The druid is without spells for the rest of the day and all they can do is observe.
Now what spell are you using to create wind to blow out the campfires? At 4th level you're really limited on what you can take.
I'd also like to point out that if a druid cripples the supply train for a major army then he paints an enormous target on his back. Not only will large armies have spellcasters on retainer (possibly even druids of their own) but they may have some other kind of support. They may have the aid of infernal allies (Chelaxian), elemental tribes (Osirion), or even mechanical guardsmen (Numerian.) God forbid they hire out a contract on the druid's life with the Red Mantis Assassins.
For the druid that starts to warn wildlife away from the camp then a priest will create food to feed the troops or a sorcerer will intimidate a local village into giving up supplies. These are just a few methods.
Having a druid hamstring an army is a great idea and for a starting party it's a great way for the druid to play a vital role but druids would never end war in Golarion.