bash - Reverting back to backup.txt for specific lines -

i have backup.txt

five 6 7 5 6 7 seven 6 5 five more here 5 five fivefive fivefivefive fivefivefive? 

and new.txt

four 6 7 4 6 7 seven 6 4 four more here 4 four fourfour fourfourfour fourfourfour? 

i want revert lines 3 , 4 of new text how in backup.txt

five more here 5 five fivefive fivefivefive fivefivefive? 

how accomplish sed?

so new.txt like

four 6 7 4 6 7 seven 6 4 5 more here 5 five fivefive fivefivefive fivefivefive? 

in specific case

sed -n '1p;2p;7p;7p' new.txt backupt.txt  

will work expected.