50k to spend

A few others I would take a look at-

Acura TLX SH-AWD Advance Package- stickers for under $46k, handles great thanks to SH-AWD (Super-Handling All-Wheel-Drive) and has plenty of kick thanks to the 290hp 3.5L V6 and 8-speed A/T.

Volvo S60 T6 Drive-E FWD- I just bought one of these and it had a sticker price of just under $46k and I paid $41k and change. I got the Platinum trim level and added 19" wheels, Blind Spot Information System and Heated Seats. It has a 2.0L Supercharged AND Turbocharged ("twin-charged") 4-cylinder that puts out 302hp and has the best automatic transmission I've ever experienced (an 8-speed with lighting fast up/down-shifts at anything more than the half-throttle. 0-60 in 5.4sec according to Volvo but some auto reviewers have managed to hit 60mph in as little as 4.9sec! Always has tons of power on tap and will push you back in the seat at any speed with a press of your right foot!

I'd also recommend taking a look at a few different models from manufacturers that you mentioned-

Instead of the Audi A4, I find the A3 to be a much better looking and better driving vehicle. It's also slightly less expensive which only sweetens the deal.

The BMW 3-series is undeniably a great car. But there is an even better BMW built off the 3-series platform that is much better looking and far less common- the 4-series 'Gran Coupe'. It's the sexiest 4-door that BMW makes, IMO.

Within your list, the Cadillac ATS is the most fun to drive, while the Mercedes is probably the best all-around car. It's essentially a compact S-Class.

The Lexus IS isn't the best at anything, but if you like how it looks the rest is quite good. And it has more rear seat legroom than the Cadillac or the Mercedes.

I'm not crazy about the latest 3-Series. If you want more space and utility without giving up a compact feel, though, the 3-Series GT hatchback fills a unique space. It's a little longer, taller, and wider than the regular 3, for about the same price.