OK, I finally completed level 3 (thanks to TCD for the tip with obtaining time bonus which helps immensely!) and here's a snapshot of level 4 (I didn't bother with the usual lo-res/centre screen options here...):
I need to figure out how to use WinUAE record video mode (all these years using WinUAE and never used this section) as would had best showed how I completed it (tutorial anywhere?). But, in a nutshell, move like the wind! There is little margin for error on level 3. Once you get to the middle of level 3 (where the exit is), you simply "build" a bridge over the dragons.