This is a scalpel blade holder that should fit the Leatherman Surge holder, and other models with a similar holder. If you have a standard no 4. blade, use the file version3
Here are the instructions from DrPorkster on how to install and remove the blade, I find them really helpful.
"To attatch the blade, align the centre oval slot of the blade with the holder and using your thumb nail apply slight pressure to the blunt end until it slips into the grove at the same time as making sure the sharp end is in the front tip groove.

To remove the blade, simply apply a little pressure to the underside of the blade by the oval locating hole and it will pop off.

If you need to apply any pressure at the sharp end of the blade use a block of wood and keep your fingers away."

The scalpel blade on the picture is only for visualisation. Be careful around blades, I accept no responsability for any injury's.

I tried to print this, it's really flexible and you cant apply pressure without it bending, so I will try to make it thicker, and maybe make hole throughout the length to put something like an unfolded paperclip for added rigidity.

Credit to this person (https://www.shapeways.com/product/3KMEHP92L/leatherman-t-shank-no-4-scalpel-holder) that gave me the idea to try and design one by myself.
Also check out this person DrPorkster (https://www.thingiverse.com/thing:1346207), because I took the blade holder from his model.