If I use 4oz of watermelon extract at bottling time,do I have to cut back on the amount priming sugar to prevent bottle bombs.
The ingredients you posted on the other question
Orange- oil of orange, alcohol, water. Watermelon-propylene glycol, alcohol, water, natural watermelon flavor
don't contain any fermentables, so you should add priming sugar also.
Go easy on the amount - 4oz in 5 gallons is the upper limit what you need, so you could start with less than that, e.g 3oz.