If you open your ES2 forms in ES4 and save them, it is most likely going to upgrade the XFA version of your forms, so that the forms work smoothly in latest Acrobat / reader releases as well.
Along side, that also makes your forms compatible with the latest LiveCycle Server release i.e. ES4.
when you import your forms, you should ideally use the REPORT section in designer to see if there are any warnings, which show up in the ES4 designer, which are not there in ES2 designer.