The facing shouldn't be there in the first place. The facing/vapor barrier should always be on the inside. ("Warm side")
Having the facing on the outside creates a moisture pocket - moisture that gets between the ceiling below and the facing above becomes trapped, and encourages rot and mold growth.
Remove the facing and then you can pile on more insulation to your hearts content. Lay it perpendicular to the existing layer.
Since you are in zone 4, R-38 is a reasonable insulation level. That doesn't tell us how much an issue a lack of vapor barrier is. It is less of an issue in dry climates. What a barrier does for you is prevent moisture from condensing inside the insulation, significantly reducing it's insulating value. Ever notice how hypothermia victims have invariably gotten themselves wet? Unfortunately, properly installing one can be very difficult (read: expensive). As it happens, plywood acts somewhat as a barrier, depending on it's construction. Thus, you may need to just focus on gaps and openings.
Another possible make-do vapor barrier is a good heavy coating of latex paint on the ceiling. You may already have this! Truth be told, the best way to insulate would be to remove the plywood and blow in the needed insulation. This gets sealed around all the joists and other ins and outs nicely and makes effective use of what little insulation is already there. However, it may not be worth the effort.
You can still get a good insulation job by layering it on top as you suggest. The draw back is the dead space under the plywood effectively becomes part of your conditioned space, greatly diminishing the value of what's there. Even a plaster ceiling alone is worth something. And you've increased the top story volume you're heating 5-6%. Another issue with this approach is the plywood becomes your vapor barrier. What are the ramifications of moisture condensing on the under side? There's poor air circulation, so it will not dry easily. It could be the cause of a mold colony gaining a foothold, which can have serious health consequences. Or it may not be an issue if you're in a dry climate.
Whichever way you go, try to ensure what ever passes as a vapor barrier is a complete as possible. Ensure any recessed light fixtures are not insulated over, and that no insulation contacts the cans. It may seem more efficient to cover them, but you greatly increase the chance of them overheating and starting a fire. Insulation is little good if your house burns down! A truly efficient ceiling has no recessed light fixtures for this very reason. Note that there are supposedly fixtures rated to be covered, but I don't think I've ever seen one.
Best Answer
Blown in insulation fills the cracks better. For that reason, as a second layer it has big advantages.
Downside: You have to borrow/rent the blower, which, if you are in my situation, you need to plan two trips to the hardware store an hour away.
Upside: Blown insulation goes in very fast. My stepson and I did one section of my house, 28 x 40 feet in about 1.5 hours, once we were set up.
Downside: This is a really dirty job. You need to wear goggles and a good dust mask. The coffee filter cones aren't adequate for this. It also gets in your clothes, hair, ears...
Downside: It takes two people: One to break bundles and put them in the blower, one to crawl around in the attic.
Batts: My experiences with batts in attic spaces has been dismal. Lots of places where gaps were left either because of blocks nailed to trusses, or batts that were sloppily installed. Batts are for walls.
You have a choice of either fiberglass blown in, or cellulose. The latter is in essence finely shredded newsprint treated with a fire retardant. It's R value tends to be lower than fiberglass, but it's also substantially cheaper. Also is more sustainable.