I really don't know the answer to this one! I think it doesn't matter how many kids you have, they always want more time.
I guess I just try and say to myself to try and make the minutes matter.
I know that might sound sactimummy but I mean, you don't have to schedule an activity or specified time (though it's awesome if you can) but it's just trying to snag moments.
Four is a bloody tough age IMO - I call it the fu(king fours!
Maybe just try and empathise with her "I know it sucks when X gets in the way but I love you and am sooo proud of how patient you're being! I just love being your mum!" or something to that effect.
I wish there was an app!