Quadruple Sudoku (Fun With Sudoku #17)

1st episode of Sudoku Mahabharat is going to start tomorrow and this episode contains the following standard Sudoku Variations
Classic Sudoku
Extra Region Sudoku
Palindrome Sudoku
UnTouch Sudoku
Diagonal Sudoku and
Quadruple Sudoku

All these Sudoku types are already posted on this blog. This Quadruple Sudoku puzzle was created by Ritesh Gupta and his brother Hitesh Gupta for Indian Sudoku Mock Tests. This puzzle was posted on this blog in May 2010. I am re-posting this Sudoku as the 17th Sudoku of Fun With Sudoku Series so that this can be used as the practice of this episode of Sudoku Mahabharat.

Rules of Quadruple Sudoku

Fill in the grid from 1 through 9 so that every row, every column, and every 3X3 box contains distinct digits. Each set of four small digits in the intersection of two lines indicate the digits that are in the four adjacent cells.