I want to develop a 8051 system with very large lookup table. Obviously it will be more than 4K code size what will not fit in the internal FLash of 8051
Why not, there are derivatives with up to 128k of internal flash.

Switching to a derivative with more internal flash will be soooo much simpler (and probably cheaper) that rigging an external flash. Another advantage of internal memory is that it leaves P0 and P2 for I/O.

For reference have a look at the P89C51Rx2 16/32/64k internal flash. If yuou need data memory as well, have a look at the P89C66x and the Cygnal F12x.

Erik