So you want to add hops to a Quadruppel in order to take it into IPA range. Should be fairly straightforward.
It's going to be a BIG beer. Aerate well and pitch plenty of healthy, viable yeast, preferably slurry from a previous batch. Use yeast nutrient! You need to give the yeast plenty of what they need to do their job. Beers this size are not "dump the smack pack and forget" fermentations. Use a yeast capable of handling such a big beer; I like Wyeast's Trappist High Gravity - it gives a good flavor profile, and it can handle Quads.
Use European noble hops instead of American varieties, use plenty of flavor and aroma additions. You'll probably want to dry-hop the snot out of it, too. Styrian Goldings are nice, as are the German nobles. If you can get it, Strisselspalt has a wonderful aroma I find hypnotic.
Essentially, you're brewing a barleywine, using a Belgian yeast, and dry-hopping with European aroma hops.
Design a grist to produce at least 1.085. Use a relatively large proportion of Vienna and Munich malts in the grist for depth of character and to support the ridiculous hops; I'd do 1/3 Pils, 1/3 Vienna, 1/3 Munich. Use a proportion of Special B to add flavor and color, but don't go overboard. If you want it darker, you can add a tiny amount of Carafa. Do not use roasted malts other than Carafa or another dehusked, debittered black malt. Mash low, or you'll end up with a syrupy mess; no higher than 149.
Then add invert sugar or sucrose to get to 1.095 - 1.100 in the kettle. Personally, I'd use dark candisugar to get that deep amber, almost brown color. That has the added circumstance of raising the OG to the target of 1.100.
Use a high-alpha bittering hops variety with clean flavor; you want a clean bitterness for the flavors from the yeast and late-hops can shine through. Target or Magnum are both good choices. Avoid the obviously American high-alpha varieties like Amarillo, Chinook, etc.; they have an annoying tendency to leave flavor in the beer no matter how long you boil them. Also, it is possible to impart objectionable characteristics by boiling too much vegetable matter; using high-alpha varieties avoids the large quantities of low-alpha hops you need to reach a reasonable IBU on a beer like this.
Oh, and patience. You're going to need a
lot of patience. A beer like this could take two to three years in the bottle to calm down enough to be drinkable. Before that it's likely to taste like licking the flight deck of an aircraft carrier.
Anyway, that's how I'd approach the process. I can't say such a project is something I'd take on, but more power to ya!
Cheers,
Bob