Download original firmware for TP-Link tl-wr840n v4 from Tp-Link site.
Extract firmware from zip.
dd if="name_of_file_of_firmware" of=tp_recovery.bin skip=1 bs=512
Copy tp_recovery.bin in your tftp server
Set ip of the network card 192.168.0.66, with router off press the wps button for 5 second and you see tftpd server start transfer.