I'm not sure if you would notice if it was in focus or not.
Let me explain . . .
1200/5 gives a magnification of 240x. You then multiply that by 2.5 and you get a theoretical magnification of 600x. I say 'theoretical' because the image will be so dim you would have great difficulty in seeing it. An 8" telescope might have a top magnification of 50 x 8 or 400x on a very good night. More like 40 x 8 or 320x on reasonably good night. 600x in the UK isn't going to work even on a much bigger telescope.
Your 4mm eyepiece alone would give you 1200/4 or 300x, which on the face of it seems ok, but Dobsonians are pretty critical when it comes to higher magnifications. (That might be one of the reasons you had problems with the eyepiece you have) A top notch eyepiece might give you good results, but a lot will depend on the quality of the rest of the telescope and its collimation. I would consider borrowing a high quality eyepiece and trying it out before buying it.
Personally I find a 4mm eyepiece very difficult to squint through. I find an 8mm with a x2 barlow much easier. (Old guy!