I did an all-grain rye for a mash and pitched with Premier Cuvee yeast (tolerant to 18% abv), 5 grams to 4 gal wort. Repitched it again and the forced fermentation test indicates that the mash is done at 1.030. Any ideas/suggestions? Should I check and adjust the pH if it is below 4?
Wow, this brew is really giving you trouble! The high FG is most likely a problem with the mash, that it was too warm, producing a lot of unfermentable sugars, or that conversion was not complete, resulting in a lot of starch and cloudiness. Both will give you a high finishing gravity.
To try to fix this, you can add additional enzymes to break down the unfermentables into fermentables. One common source of enzymes is the dietry suppliment Beano. Another is the product known as Dry Beer Enzyme. The one time I used it, gravity fell from 1.015 to 1.002 - I don't recall the timescales, but at room temperature the enzyme needs a few days or more to break down the sugars.
One downside with adding enzymes to the fermentor is that it's difficult to control the final gravity. With hungry champagne yeast, you may end up consuming all the sugars, leading to a very dry and bitter beer.
EDIT: Regarding pH, I'd be surprised if your pH is below 4, unless there were lots of highly kilned malts and your water is soft. The optimal pH for fermentation is pH 4.6, although yeast will ferment below pH 4, just slower. In this paper, yeast continue fermenting down to pH 2.8.