Hi I ordered a carbon filter as a spare but I was still using my old one now it is time to be replaced I took it out of the packet and it isn't 4 inch maybe 6 inch so it doesn't fit inside of the plastic casing like my old one but will fit ever so tightly to the outside of casing the question is would I be better doing this or running some ducting from the one to the other I have been trying to attach it for about 40 minute's lol like I said it will be tight to the outside but won't fit inside short story I have been sent a different size to what I ordered the inside of casing is 4 inch the outside is like 6 inch.