5 Star C4 Window Pocket Envelope Peel Seal 100gsm 324x229mm White (Pack of 250) 9066405 Star white C4 envelopes. These envelopes have a blue opaque interior for security and are made from mediumweight 100gsm paper, making them perfect for everyday use. They also feature a window which allows you to clearly display the address of your correspondent. This pack contains 250 envelopes. C4 size 324x229mm Made from mediumweight 100gsm paper Window size of 40x100mm Window position 25mm in from left hand side, 213mm up Peel and seal closure