The Challenge-IV_Report.html file in the April updates specifies that levels 937 and 940 in the Challenge-IV.lvl file were changed. So any high scores that you had for them will be deleted if/when you run the "LT Updates" program (LTUpdate.exe) on the April Updates.zip file. (Very Important - you should select at least one backup method when you run LTUpdate.exe on this Updates.zip file.)
These levels were actually changed between the 2 "Issues" of the March updates - see Message #7639 (13-Mar-09). Therfore, if you completed the Issue 2 version of either of these levels (i.e., with Challenge-IV.lvl dated 4-Mar-09) you should restore their high score(s) from the Challenge-IV.hs file that was (hopefully) backed up by LTUpdate.exe. You can use the LTCopyHS.exe (Copy High Scores) program in the LaserTank Utilities Suite - see the More Downloads > Applications page.