Use the sleeping time to make food and freeze it - the sleeping won't last! But is normal now (as long as they are waking to feed and gaining weight etc etc). Sleep is good for them - it's when they develop their brains and grow.
Bouncy chairs are great, but I think there are probably limits to the lengths of time they should be in them for at 4 weeks, due to the amount of support they get. Have a look online for more on that maybe.
When they are awake, as others have said, your face and voice is great entertainment. I don't think their vision is that well developed yet, so also try some black and white stuff - there is a nice squishy book called faces, which makes a nice crinkly noise too. Watch your baby closely and see what they seem to like and just do more as they seem more alert - don't let the big industry focused on 'developing' your baby make you think you need to do more than that! 4 weeks is very little.
Agree that fresh air is good - especially as apparently natural light helps them learn it's daytime, which then helps them learn it's night time, which should eventually lead to sleep. . . . . Plus babies love to look at natural things like tree leaves (difficult this time of year)
On routine, I wouldn't worry about it too much yet. The main advice I would give is to look out for when your baby looks tired, and then get them to sleep asap - avoids over tiredness. I did a bath every night from about 5 weeks, not at a set time but at a time that seemed to work for the little one each day. For my benefit as well as theirs, to mark the transition from day to night, which could get a bit blurry with all the feeding!