The money and lives spent today will be well worth it in the future. Spreading democracy is worth it in the long run.
We were in a perpetual state of war after the first gulf war, bombings and no-fly zones, etc. The people that were against the invasion never seem to suggest what we should have done about that. Just left and not enforced the treaties from the first gulf war?
And not everyone in Iraq wants us to leave, in fact probably only a small percent want that. People in Iraq would not feel safe if they spoke out and said they wanted us to stay. They won't get threatened if they say in public they are against us staying. The people that want us to leave are the same ones that don't want freedom and want to intimidate the other citizens there.
If 4000 deaths is a lost war, then we totally lost World War II.