Nicky, the good news is that the September 13 time period will be a slow period for Tobago, since the school holiday season will be over and it's too early for the winter season . This means that you don't need to panic about locking down a villa booking immediately.
You mentioned 4 villas as the only ones to get back to you so far. Another option is to contact one of the real estate agents in Tobago. They all manage rental villas and could advise you about suitable villas that would meet your needs. I personally lean towards the Bon Accord area as both Store Bay and Pigeon Point (two of the island's most popular beaches) are 15-25 minutes away on foot (3-6 minutes by car). Plus the Bon Accord/Crown Point area has lots of food places and bars.
Here's a list of the main real estate villa agents who have web sites:
Brian Abraham at Abraham Realty
http://www.abrahamrealty.com/
Almandoz Real Estate
http://almandoz-tobago.com/im.htm
Natalie Mahabir at Caribbean Estates
http://islreal.com/
Pat Phillips at Island Investments
http://www.realestatetobago.com/
Dawn Glaisher at Sea Jade Investments
http://www.seajadeinvestments.com/
Christine Charbonne at Villas of Tobago
http://www.villasoftobago.com/
Hope this helps.
Clyde