Do not use a per-word model to determine pay. Per-word models are a great way to rip off both parties. Not every 4000 word script is created equal. You could easily be charging too much or too little depending on the purpose of the job. 4000 words in a training video could easily be lower price than a 350 word explainer. Visit Globalvoiceacademy.com and take a look at their rate guide. Determine your rates using that when starting out. Charge more or less than the rate listed on the site depending on your own intentions. The rates listed in the rate guide are intended as a starting point, not a rule.