I can't think of any reason in particular that champagne yeast and 1056 would lead to off flavors. I suppose I have heard of some yeast that can kill others, but I feel like that's not really anything we have to worry about with purchased yeasts, and was more a concern with wild yeasts.
However, if you're only 4 days into fermentation, back well away from the carboy/bucket and let it work in peace! By extracting beta-sugars I'm assuming you meant malting low, so that was a good call if you're trying to get a low FG. However, 4 days is way too early to start worrying about stalling out. Besides, with a big beer like this, what's the rush? Give it a few weeks, then come back and see where its at. You can always add more yeast later if your 1056 has crapped out, but I wouldn't start to worry yourself about it now.
Also (you'll definitely want to double check this and not take my word), but I was under the impression that champagne yeast wasn't great at breaking down more complex sugars, which might be what's remaining at the end of the 1056 fermentation. Now if 1056 stalls out due to the higher alcohol content, but if it stalls out due to complex sugars, champagne yeast isn't a guarantee to get you any lower. Just something to think about for the future.
