I had the same thing with DD - up (for the day) at between 4-5am for about two months.

TBH, I didn't really try anything to stop her - I think I was just too tired to think straight and kind of just went with the flow.

She gradually slept in longer and longer. Now she sleeps until about 8.15.

So sorry I can't offer you more suggestions, in DD's case it just seemed to be a phase that she grew out of.

But cutting back on her day sleep sounds worth a shot though I know how difficult that is to do because if you've been up very early, you really want them to sleep as long as possible durinng the day.