I am assuming tongue length will have an effect on tongue weight. I would assume that a 5' tongue would transfer more weight than a 4' tongue. Is that right? If so, would the difference be very much.

 

It's a greater percentage.
The exact amount is determined by load placement as well as axle placement.

If a trailer has 500# tongue weight and a 4' tongue on a 16' deck trailer and you extend the tongue 1 foot without changing placement of the load, you'll gain approximately 25# additional plus the weight of the tongue extension. Assuming you have a 60:40 axle placement.(as it should be)
With a 50:50 axle placement you will only gain about 5# plus the tongue extension.

Even with a 60:40 axle placement if you change the placement of the load you could essentially negate the tongue gain including the added weight of the tongue extension.

EDIT: Sorry that is as clean as I can explain it.

 

Thank You!!