File manipulation

I Have a file which has the structure as

filesystemname %usage %left Flag
/tmp 87 13 1
/tmp/kir 24 76 0
/tmp/kra 50 50 1
/tmp/kir/vin 23 77 2

I need to manipulate only the flag value. i.e., I should be able to change only the flag value. If i do need to change the flag under /tmp/kra how can I do that with out changing the other variables.

# changes '1' to '1000'
sed -e '/\/tmp\/kra /s/[^ ][^ ]*$/1000/g' file.txt