Brace yourself, I'm going to go against the flow...
By four or five weeks old DD had a routine, it was definitely HER routine, not ours, and we flexed around it. But understanding it allowed me to get stuff done around her sleeping and feeding needs.
The only thing that we did which may have contributed was that we FF her (though at this stage I was still failing to mixed-feed). So she wasn't eating the 'little and often' of BF babies, instead (slowly) taking 3-4oz every 3 hours or so.
Like your DD we got night time sleep in two batches (3 hours, feed, and then 4 hours) - we were very lucky in that respect. And she slept during the day for 30 mins every two hours (so 1.5 hours awake, 30 mins asleep).
So it wasn't a routine that we'd applied (she was and is too cynical to fall for our tricks to try to change it), but it was a routine. It lasted until she was about three months old, then she started to sleep (usually) from 10pm to 7am. She didn't drop the short naps every 1.5hours until she was six months old.
We've been lucky sleep-wise, nothing we've done has accounted for this, it's purely her nature.