I make 3.5 gal batches all the time
If you have Sam's Club near ya get this...... http://www.samsclub.com/sams/member-....ip?navAction=
Throw away 4 gal carboys!!!!!! that come filled with water
I didn't say it was your fault....I said I'm going to BLAME you!
You should just ferment in your 5 gallon bucket. The chance of something bad happening is minuscule, assuming solid sanitation practices.