I'd pay my friend for the ruined items and apologize, and tell my four year old that friend and friend's parents were really unhappy because their things now had nail polish all over them, and I'd explain (if my kid didn't know) that nail polish couldn't be washed off and so was really important to only use it with an adult and veeery carefully.
I'd also make a mental note that 4 year olds need adult supervision and that I should check for stuff like nail polish before letting my kid play in a room where I can't see her. Yeah, kid did a dumb thing, but she's four, and four year olds do stuff like that. My kid actually remembers being 3 and 4, and how she thought about things when she was that age... and it's really interesting to hear her talk about it because it's so clear that she wasn't able to reason in an adult way, and that things that seem mind-blowingly obviously dumb to an adult made perfect sense to her.
This is what I would do as well. Depending on the 4 year old, I would either make her apologize or have her there while I modeled apologizing. DD would likely become very upset about apologizing and I would tell her that it is hard for me to apologize too because apologizing is uncomfortable but it is a necessary part of having friends.