Unfortunately no .... the RCA analog in's are mono ... you need to use them both and pan them (one hard left and one hard right) to get the stereo image.
The SPDIF, on the other hand, is a stereo RCA connection (digital only).
The only option I see, for you to be able to do 4 simultaneous tracks, would be .....
A) Use two of the XLR input channels on your mixer (unless you have some other pre amps) ... one panned left and one panned right ... then you would use the tape out's to the analog in's of the Delta.
B) Purchase a two channel pre amp w/SPDIF out such as a
ART DPSII , which will go to the RCA coaxial SPDIF in on the Delta.
The only problem I see with this arrangement would be, how to monitor all 4 inputs. You would pry have to use live monitoring within n-Track.
May be time to start saving up to upgrade the sound card.
$400 will get you
this , 8 analog in's, 2 with pre amps. Up to 18 in's using ADAT and SPDIF outboard gear.
There may be other options out there for less money, but you get what you pay for. Stay away from USB options, as most have trouble with those devices.
Myself, I use a MOTU 828mkII 20 in's and 22 out's (with the optional outboard gear I have). Yet, I shelled out $1050 to get to that point. That may seem like a healthy amount of cash, but it's just a drop in the bucket compared to the $$ spent for pro studio gear, to get high quality home studio gear.