I've just purchased a Saturn 2 4X4, and it has some very significant quality issues. It was not built to the advertised 0.1mm specification. I am having difficulty with communication with FLA to try to reach some sort of resolution, so I'm not comfortable recommending any of their products at this point. Disclaimer: I am an unhappy customer, and, admittedly, am heavily biased. If you are interested, here is a link to my thread, which is still ongoing: https://www.cnczone.com/forums/finel...cnc-posts.html
If your main aim is to cut aluminum, you may want to consider getting a small metal mill. The Grizzly G0704 is a popular mill, and there are ball screw conversion kits available, e.g., https://www.automationtechnologiesin...onversion-kit/. I believe others also make a conversion, and you may be able to find a converted machine within your budget.
If you really need a 4 foot bed dimension, then you may want to consider the CNC Router Parts 2X4 pro version. I don't recommend the standard version. Keep in mind that a machine built from 80/20 extrusions and bolted together probably isn't going to be rigid enough to take more than light cuts in aluminum.
Gary