Yes, your sales price will be limited by an appraisal. I am sure most of the Realtors here have seen two appraisals come in at very different numbers on the same house. It is the buyer's bank that is going to pick the appraiser, so you won't really know what it is going to appraise for until that appraisal takes place.
As Scott said, your home will be comped with similarly sized homes nearby. The solds are what really matters, not the on market, in determining value. If your 5th bedroom is created by a strange or obsolete floorplan, then it won't be much of a benefit. Yes, you will be impacted by the 4 bedroom sales in the sub. If there are no other 5 BR sales, then that is likely what the appraiser will use, especially if the houses are similar in size.
The appraiser can go outside your sub to find comps. They'll usually stay as close as they can.
I am sure you have heard the adage "don't buy the nicest home on the block." You might experience why this is true, unfortunately.
Your agent can help you by staiying on top of the solds and and making sure an appariser and a potential buyer is aware of the comps that support your pricing.
Most commonly used is teh sales approach where they will find the 3 most similar sales wqithin the last 6 months and the 3 most similar homes currently listed in your area. Unless there are absolutely no comps, the price per square foot method is considered but not commonly used in appraising residential homes.