fermenting 4 gallons in a 6 gallon carboy?

I'm going to try my first all grain batch via deathbrewers stovetop all grain method. Because I want to brew a a weizenbock which calls for more grain than the 12 lbs that deatbrewer says will work, I have decided to scale the recipe back to 4 gallons.

I only have a 6 gallon better bottle and a bucket to ferment in. I have been using the better bottle as my primary because my bucket has a stain in it and I am suspect of the seal on the lid.

Now to my question. Can I ferment 4 gallons in a 6 gallon carboy? Will there be too much headspace to do an extended fermentation with no secondary?

You'll be fine. The gases from fermentation will fill up that space fairly quickly.

that will work just fine. as the yeast ferment they will fill the head space with CO2 protecting it from oxidation.