The issue is that IPB4 has the correct css paragraph styling and IPB3 had it wrong. In IPB4 if you want a new paragraph (i.e. the paragraph is separated by some whitespace) then you just press enter once and you get this:
<p>First para</p>
<p>Second para</p>
Perfect html. If you want lines bunched together you can use Shift+Enter to get <br>.
On IPB3 if you wanted a new paragraph you had to press enter twice to get the correct spacing so you would end up with:
<p>First para</p>
<p> </p>
<p>Second para</p>
This looks "double spaced" on IPB4 so you have two choices:
Option 1: Add the CSS above and work in the same "broken" way as IPB3
Option 2: Leave the new more-correct spacing style but accept that all your old posts will look like crap. If your users are happy that old posts look weird then it's the best way to go - but it's not pretty.
What is needed is option 3:
Option 3: Leave the new spacing style but fix up old posts by removing erroneous <p> </p>
It's not that easy to achieve though. Once you start having posts with multiple blank lines together it's tricky to process. If IPS didn't already consider this for the upgrade routine then I doubt they will now. I'm not sure if I can be bothered to code it as a custom mod for my upgrade either.
That's the way IPS works now. There is no much i can do about it. IPB 4 is designed and programmed for modern OS's, browsers, resolutions etc. Only issue I have is paragraph spacing.