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For the Lee filter holder system, including the Lee lens hoods that have filter slots, remember that you can reconfigure the holder to accomodate 4mm filters by changing out the filter guides. You can put between one and three slots on Lee holders, and each slot can be for any of three different filter thicknesses, 1mm, 2mm, or 4mm. The Lee side guides, and various lengths of the special Lee screws, are all sold through various Lee dealers, including B&H Photo. Note that the screws are not a common thread size, so go ahead and get the Lee screws of the length you need.
For very wide angle lenses like the Canon 17mm TS-E described in this thread, it can be very helpful to change to just one slot on the Lee holder, to give you the best chance of nothing showing up in the image.