I very much agree about abonding the naughty dtep it seems to exhaust its self around 3 or 4 years old, when they realise it can and will become an exhausting battle and rightfully so when talking and reasoning is a skill to be devolped at this age. And when that fails a more punative nature that has a meaningfull impact is the way for ward.
I progressed from the naughty step with a brief and clear explination as to why a certain behaviour etc was not appropriate, would give dd a warning if it continued then, for instance no playing in the bath, it would be bath supper to bed with no time to play.
I would keep it fairly consistant for a while so she understood a clear puntative action. so if she missed behaved after dinner time it would be no playing in bath. If she miss behaved in the morning, then fav tv programme or trip out would be stopped.
They soon begin to learn how a break in routine would be detrimental and so a warning tends to be enough to either get you both in a possitive frame, I often would say to dd and that would be a shame as we have so much fun at bath or at the library and it gives a chance to break the negative cycle.
give your warning and walk away if she has lost control of her ability to regulate her own behaviour, if she open to reason, be tactile and loving in your warning, distract her from the negativity.
I also feel that this a very normal stage of development and probably has very little to do with a new baby, it has coincided with it yes and possibly impacted on her but, I definatly seen this regression etc with my own dd and the strenghth of will yoiu describe, at 4 she is learning to regulate her own behaviour, actions and emotions and its pretty exhausting so no wonder she has slightly regressed. I would also help her out a bit and help with putting on pants or coat, pick your battles, there is no point getting to the stage you both did that resulted in you dropping her off to someone else, that achived nothing for either of you. compromise is a skill you need to teach her so why not start with small compromises like helping with pants when she has a nursery session ahead and she is delaying it. ill help with your pants but you must put your books away, get your shoes, take your plate to the sink etc as we must get to nursery on time and that would be very grown up if you could manage that.
at 32 wks pg you really dont need to be battling with a 4 year old over pants 