There is plans for a "Union West" at Pearson. Extending Sheppard to it would make sense. After all, there are 2 "lines" to Union. Realistically, just going to Sheppard West would be enough to make the line useful enough.
As I think about it, Line 4 is potentially very useful up to Jane, where it intersects Line 1 and the Downsview redevelopment, and Jane St. and a future Jane rapid transit line. Line 4 extensions further west will be through hydro lands, ravines, low density suburbia and industrial lands, which may or may not have redevelopment potential. It would be a high cost to bear for an area that is, for now, not offering much ridership potential. It's a worse corridor than the Eglinton west corridor, and many times more expensive than the potential Finch extension.
If somehow, the ridership on Line 5 and Line 6 are far over capacity, then extending Line 4 further west would be reasonable. But, the Finch and Eglinton corridors has about 40k-50k ppd each. I'm not sure what Sheppard has, but it's somewhere under 30k for busses (+50k for Line 4). An optimized subway line can push up to 1000k ppd (Yonge has about 800k ppd now). Honestly, if the LRT's are overcapacity, it's not going to be anytime soon, assuming they can move between 30-50% of the ridership of a subway line.