Nope, it's not too much. You also won't need to worry about this filter shooting out all this water through a single high speed jet output nozzle. This isn't going to turn your tank into a freshwater fishy jacuzzi... lol. Rena Filstar filters come with a spraybar outflow system that diffuses all the return water flow over a wider area. You also get to decide whether you want that spray to come about above or below the waterline. So this gives you a lot of control to make your own adjustments.
Back to our two estimating methods again...
1. 47 gallon tank x 4 = a filter rated for a 188 gallon tank.
2. 47 gallon tank x 10 = 470 gallons per hour.
Using both methods, the XP4 will be slightly overfiltering the tank. And that gives you a great margin to work with.
Keep in mind that both of these methods are complete approximations that we use to estimate the ideal (or at least a sufficient) level of filtration we need to keep up with the bioload goldfish produce. It's really the best we can do without actual load calculations and actual performance results to go by.
When I first got into fishkeeping (mind you, this was for general tropical community tank setups)... I was taught to take the tank size and double it when choosing a filter. That's because the so-called manufacturer performance ratings on filters are terribly over-exaggerated. Those are sales/marketing numbers that get printed on the box to sell us the product. In real life, filters never actually perform anywhere near those advertised recommendations or ratings. So getting a filter that's good for twice the actual tank means if the REAL performance is 1/2 what the marketing guys claim.. you're still covered. So it's a safe bet, in other words.
Later on when I started keeping goldfish, the same person suggested I take that "doubled" filter rating/capacity for the size tank I'm using for tropicals... and "double it again" if it's being used for goldfish. That's where I got my "4 times the tank size" rule of thumb a very long time ago... and it's worked fabulously for me over the years. More recently, I have learned the more reliable goldfish "experts" I've come to rely upon have been making the "10 times tank volume" method to determine how many gallons of water needs filtered per hour. Does it matter which method you like using? I don't think so... seems to me they both work consistently well.
Now that you're thinking of getting this filter, I'll complicate things a little more by pointing out that you don't have to rely on just one filter to do the job. It doesn't matter if you have one solitary filter circulating 450 gallons of water each hour... you could have two filters running together that (combined) will do the same thing. As tank sizes get larger (usually around the 50 gallon size and higher), you'll notice that a lot of fishkeepers would rather run a primary filter and a secondary filter together. This approach offers some added benefits and protections. If one filter goes out, there's still something running temporarily which buys you time to either make repairs or buy a replacement. You can also take one filter down for a thorough cleaning while leaving the other one running to lower your risk of wiping out all the beneficial bacteria that lives in your media (causing a mini-cycle).
