Disclaimer: I've been here like 4 times so I'll try and give a compilation of my experiences.
First off, it is impossible to find parking in Streeterville on a weeknight. I had to park all the way by Lurie (5 blocks away) since that was the closest spot I could find after about 20 minutes of circling. The bar is set up with a very large counter for sitting, a few booths, and a cozy pub table setup next to a fireplace. There's good TV coverage for sporting events, and they have a little outdoor seating. I've sat at each possible arrangement, but this time we had a lengthy table off to the side since our party was like 30 people (Arif's kind of a big deal).
My service has always been efficient and accurate. There were a few stumbles with the large group order this time, but it's usually spot on. One deal that they have which is great is there beer flight where you get four 5oz drafts for $10. The special part about it is that you can get Delirium Tremens as one of the beers which usually costs an arm and a leg. In general I'd say that the cost is a little too high. The entrees are around $14 and sandwiches are around $10. I guess it's not that bad, but I just hate it when pubs charge a bunch for their food. This time though we went for reverse happy hour where you can get a ton of menu items for $5 from 9-11PM on weekdays. So I was good to go at these prices.
The first time I went here, the waiter sold me on the Fish and Chips. I thought they were alright, but not nearly up to the standards set by The Duke of Perth. This time I grabbed the Curry Chips which were some purposefully soggy fries that were doused in a tangy curry sauce. I thought these were delicious, and I'm amazed that I didn't get any of the sauce on me with how fast I ate them.
The Baked Brie
The Steamed Mussels
The Curry Chips
Our group also ordered the Jameson Chipotle Buffalo Wings, the Harp Chicken Fingers, the Bruschetta, the Baked Brie, the Steamed Mussels, the Asian Shrimp Skewers, and a variety of sliders. I got to taste a few things here and there. The wings had a nice kick, and you could tell that there was Jameson in the sauce. I thought it was weird that instead of blue cheese or ranch that the wings came with sour cream. I was impressed with the size of the chicken fingers for $5. They were also good, but not nearly at the level of the Buffalo Grove Village Bar and Grill. I didn't get my hands on the other dishes, but I still snapped a few pics for your enjoyment. I would've had more pics, but Jessie Chai was being very grumpy at the time (I suggest you all take this out on her mother this weekend at family day...just playin).
The Jameson Chipotle Buffalo Wings
The Harp Chicken Fingers
The Asian Shrimp Skewers
I think D4 does a nice service to Streeterville by providing another option for when people get tired of Timmy O'Toole's. Beyond that they've got some good TVs, an interesting and comfortable setup, and pretty decent food. Most of the dishes are solid but don't necessarily wow you. At their regular prices I'd expect a little more. If this was just about the reverse happy hour I might go a little higher, but I'm giving D4 2.5 out of 5 Pearls.