Comparing text files and adding new text

hi wondering if can have 2 text file want compare.

one final 1 , 1 temporary

the temp file have new information in , want write these changes final file need insert them in same place in temp file example if final file has

1

2

3

6

7

and temp file has 

1

2

3

4

6

7

to insert number 4 in @ line 4 mirrors temp file final file edited out side of script information needs remain intact , new lines added in correct location in file there may more 1 change in temp file great

well compare not sure order

$a = get-content ".\test1.txt" $b = get-content ".\test2.txt" compare-object -referenceobject $a -differenceobject $b -includeequal | where-object { $_.sideindicator -eq '=>' -or $_.sideindicator -eq '=='}`  | foreach-object { $_.inputobject } | out-file -filepath final.txt -width 150 

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