Ans. In type-4 matrix coding decoding, a 5x5 matrix is used to encode and decode messages. Each letter of the alphabet is represented by a number from 1 to 25, and these numbers are arranged in the matrix. To encode a message, the letters are converted into their corresponding numbers and then multiplied by the matrix. The resulting numbers are then converted back into letters to form the encoded message. To decode a message, the reverse process is followed.