If your positive was on 22nd of March, then yes you should be measuring 6+ today.
{There is always a possibility of fertilisation happening very late and the dates being out because of that, even if you knew your ovulation date, but if you got a positive on 22nd it does seem to exclude this possibility because a very late fertilisation would not have shown up on the test at that point.
This leaves two possibilities:
The first is that you just had a 'bad' scan today where it was hard to measure and things are OK but just weren't showing on the scan. This is a possibility, it does happen and people do go on to have positive scans. I'm not sure what bearing your scan measuring 4 weeks has on things- this does seem significant to me because you wouldn't normally expect a scan to show a pregnancy sac at 4 weeks.
The second possibility is that the pregnancy is sadly not viable and has not grown in the way it should. I have been in this position and in my case sadly when I was behind for dates on early scans the pregnancy ended.
I would hope for the best but also mentally prepare for the worst just in case.
Unfortunately the only definitive way to settle this is to have a second scan after a wait of about a week to see what changes have occurred. The waiting is absolutely awful, I think it is in some ways the most difficult bit of all. Do you have anyone in RL to support you?
Things that I would do in the meantime would include contacting the EPU seeing if they will scan you on Monday (yes it is not a weeks time but the EPU will probably insist on their own scans rather than using the private one) and also asking them to do rising HCG bloods on you- this should give some insight into what is going on eg are they rising or falling, are they rising quickly enough and what level are they at (as this will suggest what should be seen on a scan).