...a very blurry, small view of Jupiter with the 4mm and Barlow...
Be aware that a 4mm eyepiece and a 3x barlow at the same time will give you a very high magnification - too high! For regular Jupiter viewing I would suggest you stick to 100x or 200x at most, unless the air is exceptionally still. (After a few sessions you'll find out what "Still" air means.)
A lot of beginner scopes like this are sold with barlows that tend to give too much power, my advice is to keep it packed away for most sessions.
So I'd stick to using that 4mm on its own, jupiter will look quite small but with practice you can usually tease a bit more detail out of the image.
I don't have one of these scopes, but my experience of comparing cheap modern eyepieces with expensive ones is that the lower cost ones are generally OK these days. 4mm is quite a high power so the extra magnification will cause a lot of blurriness and it will take patience to get the focusing at its sharpest, so don't panic if everything is looking fuzzy at the moment.
For a first stage I would suggest a few things that are nothing to do with the eyepieces at all...
- Collimation - look this up, it just means adjusting the two mirrors
so your eye is looking right down the tube in a straight line. For a
long focal length scope like yours, it's not likely to be a problem
unless one of them is wildly out of line.
- Tube currents/thermal behaviour - on almost any cold night, when
the tube and main mirror are still warm from being indoors, rising
air currents in the tube will mess up your image and make it
shimmery, at high power. Low power will look OK. It might take an hour or so for the image to improve (a guess)
Anyway, in summary my guess is changing the eyepieces straight away won't make a dramatic difference. Eyepiece makers will say otherwise of course :)