Tidy up together. If you are with her, she won't have the opportunity to 'lie' about it, and to be honest at 4 she is really not going to be able to tidy on her own, she needs encouragement and fun, and doing it as part of a team.
Don't let her set unrealistic rules. One toy out at a time will never work with any child, ever!
When you impose a consequence, like taking toys away, I really think if you say something open-ended like 'you can have them back when you show you can take care of other toys' you are simply making her feel angry and powerless, which is why at the next opportunity she will no doubt do something to release her frustration....needs to be clearer, eg "I'm taking this till the morning" is enough. If you want to go down that route.
Personally, I think you should just do it together and not fight this as a battle. She's too young. Maybe if she were 9 or 10, but not four imo.
Also, you're right, she is coping with a huge family change in accepting her dad as her dad...still very new. Doesn't mean you let her get away with murder, but it does mean she needs a bit of slack, a bit more understanding, and some very clear but firm and fair rules and boundaries so that she feels secure.
best of luck; don't beat yourself up about the smack, forget it and move on to happier things.