A 4mm EP is a fairly high power one. Your scope has a focal length of 900mm so a 4mm gives 900/4=225x. In a 2.5" scope that is a lot of power. Even in absolutely perfect conditions you cannot go much beyond 30-40x per inch which only gives you say 75x - 100x maximum power, so not really surprsing that you find focusing on jupiter difficult. Remember as you increase the power you also magnify any defects, either in the optics or especially in the atmosphere. Also given Jupiter is not very high ATM the atmosphere will be playing havoc with your view.
Your other 2 eyepieces give you 900/25= 36x and 900/12= 75x which should be adequate for most targets. Also I wouldn't bother with the Barlow, even using it on the 25mm ep will give you 108x which is starting to push a small aperture scope.
With the focus jumping is it more noticeable with the higher power EPs. If so it it simply the extra power magnifying the natural jumpiness of the system. Wouldn't worry about it too much.
You will not resolve discs on stars, they are simply too far away. The star you are mentioning is Sirius which is the brightest star in Canis Major and is the brightest star in the night sky.
Don't know if you have instructions but here is a link to the instruction manual on the Meade site
http://www.meade.com/manuals/Telesco...aris60EQ-D.pdf
Malcolm