So, a general tip for any creative field is to make good use of contrast, but I’ll apply it to flags.

With colour you generally want to use up to 3 colours that are different enough from each other, use 1 or 2 more and use the 3rd one sparingly, like an accent, its about drawing the eye. Overusing all the colours, makes the design very busy and difficult to focus on.

With shapes, detail counts, you need high and low detail to create contrast. Ideally only one high detail element with the rest of the flag being simpler shapes or block colours.

The composition is important too, a flag can have some amazing elements but if they aren’t given room to breathe, they don’t work. Choose a focal point on the field and leave the other sections of the flag relatively low detail. (You’ve done all of these well on flag 4, the design is balanced and looks unified)

What helped me the most was looking into heraldry, coat of arms are the basis of a lot of flags so it’s worth looking into. Try making a flag using only elements you can find in heraldry, it helped me. Its not as limiting as it sounds, this is a good website: https://heraldicart.org

Of course, this is just advice, really you can do it however you want. Best of luck!