A little bit of background knowledge: in Linux, the default file descriptors are:
STDIN (standard input) => 0
STDOUT (standard output) => 1
STDERR (standard error) => 2
Back to the source code. To read the flag, we need to store LETMEWIN in the buffer. This can be done by read(0, buf, 32) (from STDIN => directly from user's keyboard). That is, we want fd to be 0, hence we need to pass 4660 (0x1234 in decimal) as argv.