Your patch is a bit confusing, so please correct me if I am misunderstanding something, ok?
So, [line~] can be used for several things, one of which is to control [tabread4~], other is to control a fader.
If you connect a [line~] just before [tabread4~], then you can send it messages in the form of [a, b c(, where a is the initial index to be read, b the final index and c the time in milliseconds to go from a to b.
Now if you want to fade the sound in between jumps, what you need to do is construct a fader. First of all, you can't jump immediately to the new position, you must delay the bang a little bit so that the signal can be slowly multiplied by a number that goes to 0, and then after the jump, this multiplier has to be brought slowly back to 1. By slowly I mean something like few milliseconds, 40ms for instance.
Below is a working patch that does exactly that, see if you can understand it and let me know if something is not clear. I guess you can then adapt it for your own use: