I decided to take a look at the Tascam (which I had never even opened except to get the Sonar code out of it).  I installed it on a computer and brought up Sonar just to see what it looked like.
 
Two Questions:  
1) What would be the difference between the pair of:
 
US 4x4 In 01
US 4x3 In 03
 
and 
 
US 4x4 Mixing Driver 01
US 4x3 Mixing Driver 03
 
I can choose one pair or the other.  I don't know what the difference is.
 
2)  This is a four input audio interface.  However, I only see In 01 and In 03 (as well as Out 01 and Out 03).  There are some settings in the Tascam software:
Direct Monitor Settings - Mono or Stereo for In 1-2 and In 3-4), and
Line Out 1-2 and 3-4 with options of Monitor Mix, or COmputer Out 1-2 or Computer Out 3-4
 
No matter what I change in the Tascam, I only see the 01 and 03 in Sonar.
 
 
Abacab, I think  I am going to follow your advise and run everything through the Roland AI via USB to get started when I get everything back together.

That's the way SONAR displays pairs.  When you pick an input for a track, you'll see three choices for each pair; 1-L, 1-R & 1-stereo.  Goofie, but normal
 
The Mixer versions are new from TASCAM.  Potentially higher latency than the other choice.  The driver release notes say a little about them, but not much.

Weird.   So,  if I had an instrument plugged into input 2 of the Tascam,  which of those would I choose?   

Basically, in prefs SONAR is showing you the lead channel of each stereo pair.
Here's an example from my TASCAM.  First, as show in preferences

 
When you select track inputs, you then have a choice between the left, right or full pair.
So Left IN 1 is mono channel 1, Right IN 1 is mono channel 2, etc.

I think I understand now.  If I had my Roland FA plugged in this way: Left to input 1, and Right to Input 2, I would choose the Stereo IN 1.
 
Thank you Bob.  

husker
Weird.   So,  if I had an instrument plugged into input 2 of the Tascam,  which of those would I choose?   

I think that you now have a good handle on the basics of connecting a keyboard workstation as a controller.  Any additional topics you have questions about are likely to get buried in this thread.  Except for BobF.  He's just that way 
 
You will get more responses from the community if you start a new thread for each question you have, and be as specific as possible about your question. 
 
For example, "How do I configure Sonar to work with a Tascam 4x4?".  More folks that use a Tascam are likely to notice a thread like that.

husker
I think I understand now.  If I had my Roland FA plugged in this way: Left to input 1, and Right to Input 2, I would choose the Stereo IN 1.
 
Thank you Bob.  

Yes, that's it exactly.  You're welcome.