Well, wild yeast has already taken your initial SG from 1.054 to 1.006, so at that point you had about 6% alcohol in your 4 gallons of must/wine. Now you've diluted it with water and added 6 lbs of sugar, but that alcohol hasn't gone away. Yes, it's less than the original 6%, but there is still alcohol present. Obviously it would have been easier to add the sugar initially so you could get a SG reading and determine the final alcohol value. At this point you have a more complex problem to figure out. 1 - you have your initial volume and SG readings. 2 - you added about 1 gallon of water, then sugar to exisiting juice to get an SG of 1.054. We need to know that final volume to figure out how much alcohol you will have at the end. I'm showing you should have ended up with 5.444 gallons after adding the sugar, is this correct? If so I think we can use FermCalc to figure out your final alcohol level, depending on what the final SG is.
Doing quickie math, I think the alcohol level after adding the gallon of water dropped to 4.8%. So if you ferment the 1.054 (5 gllons) to dry at .990, you would add about 8.5% more alcohol, so you'd end up with a wine with 13+ % alcohol, and I imagine a rather thin wine at this point.
Hope that helps. Someone please chime in if my figures are wrong, it is Saturday and I have had a few drinks.
I will differ to your figures, Cree, especially since I flunked Wine Math 101.

But wait...there's more!
Between crushing 50# of grapes, 95# of plums, and helping pick 3.5 bushels of apples, I sorta got lost. I failed to go over my notes after a 2 day absence.
It turns out I had 3 gal of grape juice, which was at the 1.054 SG. I topped that up to 4 gal, but failed to take a second SG reading. So...when I came back today, the 1.006 SG was probably spot on considering I'd already diluted it. THEN I topped it up to 5 gallon.

So now I have 3 gallons of grape juice in a 5 gallon batch. Ack!! But never fear! 12 lbs of sugar later I pitched my EC-1118 and nutrient in at SG 1.094.
Now, I've maintained for a couple years that a person could make wine from Kool-Aid, and the only reason you can't make Stone Wine is because rocks don't ferment. I guess we'll find out with this batch. I see about 4 cans of Welch's as back sweetening when the time comes. Store-bought frozen flavor pak. Whee! Glad I'm not a connoisseur.
But as for the OP, my 5 gallons of plum juice is hot off the press at 1.070 SG.

Cree, if you don't mind, how much sugar will I need to bring this 5 gallons up to SG 1.090?