tl dr: Not a good idea.
You can check to see if the knock sensor is going crazy through the ECU, so why disconnect the knock sensor to see? Disconnecting it will not give you any direct data. If there is a real knock problem, you could end up damaging the engine.
Second, if there is an issue with the knock sensor itself, you'd most likely get a trouble code of P0325, which is about a circuit malfunction.
our octane fuel is very bad. (but i will not rise rpm above 4000)
The top rpm doesn't matter much, as it's not about how fast you rev your engine, but rather how hard you run your engine. Knock can occur for several reasons, but it's when you hear it while the engine is being ran hard at lower rpms (below 4k) is when you do damage. Damage produced by knock can occur quickly and is irreversible (without rebuild). Serious knock can cause serious problems. Taking away the protection the computer has built into it can only lead to further issues.