We stayed at the Mitsui Garden for 2 weeks in April. There a a couple of things we liked, but also some we did not like at all. If possible we would have given a 3.5

Pros

• the staff is really nice and helpful all the time. No matter the request, everyone was eager to help us out so we could enjoy our stay
• the location is really nice, a little out of downtown but still close, subway easily accessible through the elevator, fantastic views of the city
• the room was always super clean and tidy
• the stone oven pizza was pretty good
• the interior design is pretty cool

Cons

• breakfast coffee is self service from a full automatic coffee machine. The coffee is really really bad, and it's not possible to order a cappuccino or cafe latte even though they have a real espresso machine. In fact, we never saw the espresso machine in action, not sure what it is for. It made us skip breakfast a couple of times to get a good coffee from somewhere else. Eclat des Jours Café is right in front of the hotel
• we stayed for 2 weeks and after the first few days, the breakfast became pretty dull to us. There was just no variety at all. Fruits, drinks, soup, salad - it was all the same every day, not even different on a weekend. There were also no (vegetarian) toppings to pick from, no cheese other than shredded mozzarella, just the same three jams every day. Some daily special or so would've been nice
• some of the staff barely speaks any English. This would be OK for a family hotel or a small bettei, or even for a 3 star hotel or so, but I don't think it's too much to ask for in a 4 star hotel. We got along with Google Translate and all, but it sometimes was frustrating when asking for something and you couldn't tell if you were actually heard. It may not be super important but the lack of English also kept us from having some chit chat with the staff which made the whole stay so much more impersonal. Usually when we stay at a hotel, after a few days you get to know the staff and it feels welcoming an nice, you greet each other by name and even get to know some of the staff more personal etc. Here is was always very distant
• we only had cocktails at the bar once because the cocktails were more like dull long drinks. E.g. Moscow mule was just Gin, Ginger Ale and a slice of lime. No copper mug, no deco, nothing. It felt like you would do a Moscow mule yourself, it was just not atmospheric and nice. Other drinks we had were the same, totally dull and not fun to drink at all.
• the TV is 98% Japanese. I only found CNN as the only English station. Most stations cannot be watched anyways bc subscription is missing, but you cannot rent movies either. Some streaming providers do allow access from Japan but some don't, so some way to pick a movie from a movie collection provided by the Hotel would've been nice. We also stayed at a 3 star bettei for 3 days and they did have movies for rent.
• the bathroom was pretty average. For Japanese standard, the toilet was not the best (it had no fan, no auto open/close, no auto flush). We had a way better toilet at the 3 star bettei we also stayed at. The plastic bath tub had kinda dorm charm, a large ground floor shower would have been so much nicer.
• the restaurant menu does not offer many vegetarian options other than the Pizza Margherita. There was also no plant based milk for breakfast
• no room service, no ironing service
• one day we stayed until afternoon and suddenly we heard music playing from a speaker in the room. Turns out it was the ambient background music that was played in the hotel, it was also playing in the hallway etc. It's totally fine to play some music in the hotel's public space but NOT in the guest's rooms. We had quite a jet lag and needed to rest but had to listen to music we didn't ask for.

tl;dr It's a good hotel but you should also explore other options which may better fit what you're looking for