Let me lay this to rest. For our rally team when we tow it's about 11,000 lb GVW overall with just a 4.6L F-150.
I've inadvertently towed about 2900-3200lb of a double axle enclosed with the 5EAT in an OBXT. It was not through any mountains.
I would NOT TOW 4000LB with a LEGACY. Come on. First of all, the sway. Unless you got a hensley arrow on your subie, it'll be a slow trip. The Outback couldn't go much faster than 62 without ridiculous sway. Secondly (with A/C on, summer day) the temp gauge did creep up once or twice. This just being the hills in Iowa on the way to Chicago from Denver. Note this is overall, DOWNHILL and often tailwind. I've also towed 2700 lb (scales at scrapyard) with the flatbed and NO BRAKES, across town. And I can say that if you towed that much on highway, and something happened to trailer brakes, you're ****ed. You can do as much engine braking to help out as you want, you're still hosed. I needed engine braking to stop from 25 mph at stoplights.
Now, on to towing with the ford. It only has 220 hp. Yes the subaru was better at getting things up to highway speed. The ford is a manual, and I can definitely see where getting the trailer started with a manual legacy is a problem. I used low range on some hills on farm roads etc. where I had to come to a stop.
Now let's talk about Vail Pass and the Eisenhower tunnel. Yeah sure a turbo woulda been nice to thicken that air, but a Legacy isn't a turbodiesel sprinter van. Let's talk about 18 mph and flooring it in second gear. Given that, a) most of us said we would prefer an automatic and b) it'd need the turbo cooking away, well, if it were a legacy we'd blow it up! All of it (torque convertor--> kills transmission, while engine and turbo work on murdering each other--the EGTs would be ridiculous, as I bet they already were on the ford).
Please don't do this to your legacy. A crappy pickup is quite cheap these days. The pickup was able to handle the 4300 lb of trailer and 1500 lb cargo sometimes at 78 mph (not advised). Oh and I got a taste of no trailer brakes in the mountains with the Ford. It can happen. That was already a disaster in waiting. It would be worse with a mid-size car.
Takeaway message: 3000lb is plenty. Ignore the European specs. I saw them too, they went straight to my head, and eventually out my ass.