I'm really sorry that you are going through this OP. I remember how devastating it is, our DD's measurement was similar and we'd also had a miscarriage shortly before. Our story is a very happy one though
A higher than normal measurement can be an indicator of chromosomal problems like Downs. The NIPT uses placental DNA from your blood to check for Down's, Edwards and Patau (I think some also test for Turner's if the baby is a girl, but I'm not certain). This doesn't rule out other rarer chromosome anomalies and it isn't considered diagnostic (rarely could give false results) but it generally very accurate. You could also opt for CVS now or amniocentesis in a few weeks which could check the full microarray for chromosome disorders (but this does not look at or rule our genetic disorders which are slightly different - it's impossible to test for all the possibilities but this is the case with every pregnancy.) CVS will give you a faster result but uses placental DNA so is very slightly less accurate than amnio which uses fetal DNA but both are considered diagnostic. These invasive tests (unlike NIPT) carry a very small risk of miscarriage.
If a chromosome problem is ruled out, as it was for us (we had CVS at 13 weeks) your hospital may suggest checking for a structural problem that could have caused a higher than usual level of fluid. We were sent to London for a fetal heart scan twice and had extra anomaly scans which were all normal.
At 4.1 it is higher than normal but on the lower half of the risk charts if that makes sense so if the chromosome tests and any additional scans are normal your hospital will probably say that the risk of problems with the baby are now a background risk like with every pregnancy and not to worry (so much easier said than done!)
I was so so worried throughout and spent hours and hours looking for stories and found lots and lots of posts from people who'd had a similar measurement and their babies had been born absolutely healthy and grown up to be lovely normal children with no signs of any problems.
Our DD is an amazing, happy, active little thing hitting all her milestones (walking, talking etc) early and she is the best thing ever. We found out not long after her birth that she did have a very rare physical problem which sounds severe and of course it is a big worry, but our hospital are confident surgery will be very successful and it hasn't affected her development at all - in fact most people are astonished to hear it as you'd never know to see her. We really hope and are quite confident it won't impact her life. It might have been a complete coincidence but I think it makes sense that the measurement is connected.
Please PM me if you want to talk. I remember how awful it is and will be thinking of you.