Since you are an extract brewer, you should check the recipe. Most I imagine are less than 3 gallons, probably 2. This is especially true if you are steeping specialty grains. HB99 goes with 1.5 IIRC.
There are a number of reasons for this: post boil O2 content, the difficulty in cooling a larger boil volume, etc.
You should also look into late extract addition, but perhaps after you have one or 2 batchs under your belt.
Whats the first batch going to be?
ETA: Go lower. You will get boilovers with 3 or more in a 4 g pot.