Yes you can. The front lines are 19m and back lines are 22m so the front line split is 3m up which still makes it a relatively low Y split bar.
It's fairly straightforward:
1) Remove the metal leash ring off the red safety line at the QR.
2) Pull out the red safety line from the QR and bar.
3)Disconnect the red safety line from the 5th line and wind up the 5th (save if you plan to use the bar in 5line mode or as a spare front line).
4)Use a pliers or a screw driver to pull out one of the metal retaining squares from the black splitter that holds a side front line. This line will become your new flag line.
5) Remover the metal retainer from the line and the piece of tubing just above the line.
6) Insert the front line back through the splitter and attach the red safety line to it.
7) Re-thread the red line back through the bar and QR
reattach the metal ring to the red line.
Pics below. First one is how the splitter looks 4-line mode and bottom one in original 5-line setup. I have a video of the process somewhere too...
Should only take about 5min to change it to 4-lines.
No guarantees with this mod! Make sure to test if the safety/flagging properly works with your kite and also check for wear and tear periodically.