Hi Bo,
Firstly 4 mins is OK, I have only recently dropped back from 4 min to 3 min as my new camera can rack up 10800 frames in that time and it takes forever to process.
That aside there are several suggestions I can make.
1. The images are a bit under exposed. Perhaps try upping the exposure a notch (1/60th??)(you may have to lower the gain as well which won't hurt), if your capture program has a live histogram display then use it to fill the histogram to about 80% full (on the horizontal or X Axis) or numbers in the low 200's as a guide. The brighter onscreen image will probably help you to focus as well. If your capture program doesn't have a histo display one such as wxAstroCapture does.
http://arnholm.org/astro/software/wxAstroCapture/
(Personally I don't use this for capture, just to set the exposure. Once set I would exit wxAstroCapture and reopen my normal capture program which retained all my exposure settings)
2. Try to stack several hundred frames, at 15fps if this works OK for you there should be 3000+ frames to choose from
3. Next is the hard bit. Images can lack sharpness for several reasons. Poor seeing has been the bane of most planetary imagers in Australia for months now, and shooting at a little over 30 degrees elevation as you must have been, possibly didn't help. So don't be too dissapointed if the images are not as sharp as you hoped. This type of imaging is very demanding of seeing, and even what looks good to the eye can be bad news for the camera. Usually poor seeing can be identified on the screen as wobbling and/or fuzzyness that usually comes and goes. The worse it is the harder it is to judge the best focus. Electric focusers are highly recomended but not essential, once you have used one you won't want to be without one. It is just so much easier to judge focus without the image jumping violently at each touch. Take a bit of time to focus and try to focus regularly through the session, even if you think you are good at focusing, you will improve a lot with practice. Collimation and good quality optics are also important, if you don't know how to collimate your Telescope then take the time to learn. Generally smaller telescopes seem to hold collimation better, so once done properly it should stay fixed as long as you don't have mirror flop problems. Poor quality optics can be difficult to isolate and are not confined to the cheaper brands, I use a fairly cheap chinese 2.5X Barlow, and am very happy with it. Star testing on a good night (whilst collimating) will give an idea of the quality of your outfit. I would be surprised if your scope quality was to blame in this case though, and think the likely culprit is seeing or focus or both
