At 17 shes probably still at school, likely has little support etc. 4 years down the track she could well now have finished school, and have finished uni, so she could have a degree, and a job, and therefore a reasonable income. And at very least, a degree to fall back on when she wants to go back to work. Not only that but she potentially could have been with this person a while, maybe even have got married etc. 21 is HUGELY different to 17 I recon. And 4 years is a fair amount of time! Even if she hasnt been to uni to get a degree or whatever she may have been working for that period instead and been saving money so would have better financial situation than at 17.
I dont think there should be any amount of time that you have to wait before you have more children. Its a decision for that individual. It would also depend on the reasons behind the adoption, for example if I was young and no income etc and adopted out my child then got pregnant again straight away that would be pretty silly, and yes, I probably should have waited. If however I have always wanted a child, I was in my late 20s, happily married etc and then was raped on the street and fell pregnant. If I didnt believe in abortion so went through with the pregnancy, but couldnt face the thought of looking at my child every day and being reminded of what I went through etc and adopted it out instead. Then my husband and I fulfilled our dreams of having a baby, I certainly dont see how that could be wrong in any way (I mean, I might not want to get pregnant that quick had I been raped but as an example I mean why should I be punished and not "allowed" to have another child coz I gave that one up for adoption when it occurred through no fault of my own?!)
Anyway, extreme example I know but think it makes the differences in circumstances obvious and how the situation could play a huge part in a decision either way. That said tho, I think 4 years later is quite a while later (I mean, wtf - why would she have to wait til the child was a teenager?!)