Being confined to 4 keys, but them acting up together as a group, sounds like a hardware problem. That sounds like either a flaky connector in the keyboard, or flaky electronics in the path the key switches take to to get to the processor, they might do a matrix thing with the keys in groups of 4.
Does smacking on it make it work at all? Like the keyboard section, more than the fold up synth section? That could mean it is just a flaky connector or cold solder joint.