@dawid It depends on what you need. Le Potato by Libre is somewhere between a pi 3 and 4. Orange Pi makes several boards with different cpu, ram and emmc. Rock Pi makes several boards, some more powerful than a pi 4.

Keep in mind that you'll need to run non-raspbian distros on these which may complicate things, again depending on what you're doing (gpio comes to mind). Documentation won't be as robust in many cases, etc. But the boards are readily available for a reasonable price.