I don't have a fifth strap in my ship and came up with a way to improve the effectiveness of the stock shoulder belts.
I installed a big D ring at the anchor point of the lap belts.
An additional strap was constructed with loop at one end ... it is used as shown in the photo.
This is a photo of the prototype and we have a lot of extra belt material - you can adjust for your specific ship to keep it neat.
You can pull the belts very tight and they hold tension well ... pressure on your shoulders is close to that of a crotch strap (so tight it hurts)... but not quite as neat as you do have two extra pieces of webbing. Everything releases if you have to exit. So, perhaps a reasonable solution until you install that fifth belt.
http://designtekk.com/photos/RidgeStrap.jpg
We also tested putting the D rings on the lap belts - the part you pull on to tighten. This worked fairly well if the rings were located close to the slider ... plus the arrangement seemed to keep the lap belts tensioned.
I have some extra rings and webbing from the prototyping session and would consider supplying at cost ... or you can DIY.
KK