A general rule for BIAB is to use a kettle that is twice the size of your target batch. A little bigger is even better. There are ways to make smaller kettles work, but 3 gallons into a fermenter in a 4 gallon kettle will be a bit tough. You would need to add in a sparge, and then start your boil with your kettle near the rim (or take the efficiency hit of diluting a higher OG wort).
2 gallons in a 4 gallon kettle is more straight forward. This is the size that my girlfriend brews with her 4 gallon kettle. I brew a mix of 2.5 gal batches in a 5 gallon kettle and 5.5 gallon batches in a 10 gallon kettle.
I agree with the advice to skip the Decoction. Some say it makes a difference, but in a flavorful beer like a Hefeweizen, the subtle impact would be hard to pick out. Some say that adding Melanoidin Malt at around 2% of the grist will add some of the character from the decoction...that was my plan for a Dunkleweizen I planned to brew about a year ago but never got around to it. But skipping both the decoction and melanoidin malt is fine.