It's always hard when kids change their nighttime routines on you, isn't it?
Are you sure she is tired at her normal 'bedtime'? If my girls have slept late in the morning for some reason, I need to move their bedtime back, or I tend to have problems.
She also might just be at a stage where she needs a bit less sleep? It might be an idea to wake her up 30 minutes earlier each morning and see if that helps.
With my 3.5 year old, we don't insist that she goes to bed as soon as we leave her. She has her usual bedtime routine (read stories, talk with Daddy, then nurse with Mommy). After this, sometimes she'll roll over and go right to sleep.
Other times, she just isn't quite ready for whatever reason. So she plays quietly in her room until she's ready to put herself to bed. Sometimes she'll come downstairs and ask for water, or to go to the bathroom, or something like that, but usually, she just plays for 5-10 minutes and then goes to bed.
We put in a child lamp (from Ikea) right over her bed, so she can easily turn it on/off herself...this gives her the freedom to play for awhile, then go to bed.
I think it's just a control thing...she likes to decide for herself when exactly she is going to lie down and go to sleep. We always try to make sure she is good and tired before we put her to bed. And I think she generally is tired...when she plays, it's rarely for more than 5-10 minutes, so I think it's more a 'I'm deciding when to go to bed' thing than a 'I'm not tired, so I'm going to play' thing, you know?
Would that work for your 4 year old?