S4 Engine Break-In Lugging?
 

I just picked up a new S4 earlier this week. As part of my break-in procedure I've been trying to keep the RPMs under 4k and have been trying to vary engine speed/load. I was on the highway today and was doing some mid-throttle pulls in a higher gear to load the engine and accidentally upshifted too high (8th gear at around 50mph) and experienced some engine lugging for a few seconds. How damaging is lugging to an engine during the break-in period if its only for a few seconds?

Sorry to hear. My advice drive normally. I don't see why there will be a problem. The engine and the transmission had been warmed up and properly lubricated if i suppose you have been on 50 mph.

I wouldn't worry about it too much. Honestly think the engine can take most anything for short bursts during the break in period. Just good practice to take it easy until it's broken in.

FWIW: I've never followed the break in procedure for ANY of my Audi's since 2015.
I've used the MotoMan break-in technique and have had ZERO engine oil burn issues, and frankly, the car was significantly more reliable than people that followed the factory recommended option.

These engines are manufactured to extremely rigid tolerances these days.
Break-in, in theory, for older cars was to allow the rings and other seals to acclimate as they couldn't manufacture even close to today's tolerances.
That being said, by actually driving it like you stole it from Day1 (which they do to test the engines at the factory anyway), the rings actually seat in position better and immediately become rigid in that position.

There were MANY discussions about this in B8/B8.5 threads of the past.

In fact, my 2nd Audi was a 2013 Certified S4 with 700 miles, which had a serious oil burn issue that didn't meet "Audi's" standard for the "deep dive" into a fix.....
I eventually traded that for a brand new 2015 S4 custom from the factory.
Day of delivery, I let the car warm up at idle, and then redlined, engine braked and repeated this for about 4 miles on the freeway.
40K miles later, not a single drop of oil burned and the engine ran smoother than my previous car.

Search MotoMan break in for more details...

Lugging in general is an issue given LSPI and the lack of a real lack of anti-lspi ratings on 508.00 oil. It's not a problem that you should spend a second of your life overthinking (I'm guilty of this, a lot), but try to avoid lugging especially on these EA839 engines.