Welcome to the forum
Your scope is more than capable of viewing Jupiter, just don't expect to see Hubble type images.
A few points:
- With 5" aperture the maximum magnification on a really GOOD night will be 250x which is what your 4mm gives.
- I would bet that the 4mm is not a great eyepiece so you can use it as a paper weight.
- You can also bet your 3x barlow is not great, another paper weight.
- So what to do?
- Your 20mm (50x) will perform quite well as low powers are much more forgiving to quality so its a keeper.
- Get a good 2x barlow like an Orion Shorty Plus - good investment and can be used with future scopes.
- Get two other plossls - 15mm (66x) and 12.5mm (80x)
- These can be doubled up with the barlow to give you a range that can be used on most nights.
I know this sounds like a lot of money but for starters GS eyepieces and barlow will work quite well and show a marked improvement.
How to use:
- Put scope out for an hour before planning to use it.
- Observe on a grassy area to help stop thermal interferring with your views.
- Check collimation.
- Most nights your scope will work best below 125x so the 15mm and barlow will be your workhorses.
- Going up in power will not improve your view if the previous power is not crisp. Better to back off to the next power down, a small crisp view is way better than a large blurry one.
Enough preaching, hang in there, Jupiter is not as easy to observe as you would expect but you should turn up the equatorial bands. Saturn will really hook you and you should get some great views of Omega Centurii and 47 Tuc.
Keep looking up.