This cross pendant should fit a 3mm to 4mm chain but 4mm would be very close with some chance of chain not fitting the cross, so we would recommend making the bail a bit larger. If you are ordering this diamond cross online please make a note during checkout in Additional instruction field that you need it to fit a 4mm Gucci chain, and we will adjust the bail prior to shipping the cross. Or if your budget allows you can take a look at a bit larger cross here: https://www.itshot.com/10k-gold-round-diamond-cross-pendant-1ct which we think will work a bit better with 4mm chain.