Determine the values of $n$ for which an $n\times n$ square can be tiled with pieces of the type  [img]http://oi53.tinypic.com/v3pqoh.jpg[/img].
1. **Determine the necessary condition for tiling:**
   Each T-tetromino covers 4 squares. Therefore, for an \( n \times n \) board to be completely tiled by T-tetrominos, the total number of squares, \( n^2 \), must be divisible by 4. This implies that \( n \) must be even.

2. **Color the board and analyze the tiling:**
   Paint the \( n \times n \) board in a checkerboard pattern with alternating black and white squares. Each T-tetromino will cover either:
   - 1 black and 3 white squares (type-A piece), or
   - 3 black and 1 white squares (type-B piece).

3. **Set up the equations for the number of black and white squares:**
   Let \( x \) be the number of type-A pieces and \( y \) be the number of type-B pieces. Then:
   - The number of black squares covered is \( x + 3y \).
   - The number of white squares covered is \( 3x + y \).

4. **Equate the number of black and white squares:**
   Since the board is evenly divided into black and white squares, we have:
   \[
   x + 3y = 3x + y
   \]
   Simplifying this equation:
   \[
   x + 3y = 3x + y \implies 2y = 2x \implies x = y
   \]

5. **Determine the total number of squares:**
   The total number of squares covered by the T-tetrominos is:
   \[
   n^2 = 4(x + y) = 4(2x) = 8x
   \]
   Therefore, \( n^2 \) must be divisible by 8, which implies that \( n \) must be divisible by 4.

6. **Verify the tiling for \( n = 4 \):**
   A \( 4 \times 4 \) board can be tiled with 4 T-tetrominos. This can be done by subdividing the board into smaller \( 4 \times 4 \) sections, each of which can be tiled by T-tetrominos.

7. **Generalize for larger boards:**
   For any \( 4k \times 4k \) board, subdivide the board into \( k^2 \) smaller \( 4 \times 4 \) sections. Each of these sections can be tiled with T-tetrominos, thus the entire \( 4k \times 4k \) board can be tiled.

Conclusion:
The values of \( n \) for which an \( n \times n \) square can be tiled with T-tetrominos are those for which \( n \) is divisible by 4.

The final answer is \( \boxed{ n } \) is divisible by 4.