I have two files on a linux machine:

$ cat file1.txt line1 line2 line3 line11 line12 $

$ cat file2.txt line4 line5 line6 line7 line8 line9 line10 $

I want to insert the whole contents of "file2.txt" into "file1.txt" specifically at line#4 so that the final contents of "file1.txt" become:

$ cat file1.txt line1 line2 line3 line4 line5 line6 line7 line8 line9 line10 line11 line12 $