My bf bought me a telescope for my upcoming birthday even if I'm not a guy (women don't tend to be into this stuff but I am). Cost £129 but it is a Sky-Watcher Explorer-130 EQ2 Telescope. I am still figuring out how to use it and I signed up to a stargazing forum to get some help.
I bought a cheap telescope & was lucky enough to view the Milky Way. Mars was as clear as a bell. Galaxy was perfect until I unwrapped it & ate it.
Wilkinson's are very good. I buy my cleaning products and binbags from there at a very reasonable price. Heat added.
Divide the focal length, 900mm by the eyepiece focal length so 20mm will give you 45x and 4mm will give you 225x. If you add the Barlow you will get 135x and 675x. The 675x will be unusable but I bet you try it :) Your focuser is probably a rack and pinion (the same on most scopes) but it doesn't give fine adjustments, you will struggle at high magnifications to get things in focus. I try and focus on a star before turning to a planet.
20mm works great, 4mm you struggle to focus a little, what’s magnified more 20mm or 4mm, I have a feeling it’s simply too close to focus so need to wait for the moon to be visible ;-(