I got 24 Volts boosted from Vin,
I better put a disclaimer about my web site. I have had it for many years (over 15 I think)
"I would suggest that the Uno's reg has such a diode and that the USB 5v is passing through that and losing ~.7v in the process, hence the 4v3 on VIN."
"Are you saying that your SMPS is running off the VIN 4v3?"
"Is there a connection via J1?"