Is anyone aware of a way to adapt a three on the tree shifter to a four speed transmission?? I have a four speed dodge trans and adapter bell housing that I would like to install in my 48 Plymouth, but I would like to keep the shifter on the tree.

Nope.

I seem to recall reading that maybe Volvo?? on some other overseas make may have done something like it awhile ago...

If my memory serves me right a friend of mines dad had a late 50s or early 60s sabb with four on the tree. another idea would be to use a normal three speed column box to shift the four forward gears then use an aux lever mounted somewhere out of sight to operate reverse. For reverse you would have to shift to neutral then engage reverse, could use lever or a cable to control reverse. I don't remember if a 48 plymouth used a select type shifter or a regular one like a chevy,but if nesscary you could adapt a chevy shift box to your column.

I had a 60's Econoline van that had a 4 speed column shift.

http://en.wikipedia.org/wiki/Manual_transmission#Column-mounted_shifter

Later, European and Japanese models began to have 4-speed column shifter and some of these made their way to the USA.

However, the column manual shifter disappeared in North America by the mid 1980s, last appearing in the 1986 Ford F-150. But in the rest of the world, the column mounted shifter remained in production, and was in fact common in some places. For example, all Toyota Crown and Nissan Cedric taxis in Hong Kong had the 4-speed column shift until 1999 when automatic began to be offered. Since the late 1980s or early 1990s, 5-speed column shifter has been made in some vans sold in Asia and Europe, such as Toyota Hiace and Mitsubishi L400.