Hi, my name is Mike & I have an Anet ET 4 3d printer. I had some issuses with the hot end, some of it was my fault, banged into the build plate with the nozzle ! Anyway I dissassembled the hot end, placed the heater block & radiator on a piece of aluminium on the stove element to soften the burnt on filament. With the aid of a wire brush & other tools cleaned everything up. With all the crudd cleaned off I started to inspect all the parts. I tried to push the boden tube though the extention tube but it was really tight at the threaded end. What I think was happening was when the extention tube is tightened inti the heater block it compresses the threaded area thus reducing the inside diameter. With great care I ran a #21 drill bit ( 0.159 ) 0.5 of a mm lager than 4 mm ID . ( This was done with the extention tube tightened into the heater block ) I have Capricorn tubing on order , so I’m hoping this will fit okay.
Oh the face of the tubes threaded end make sure this surface is smooth , no burrs ! I will post more when I have it back together. Note ! A tip those anoying little clips that hold the glass bed on, you know the ones you smack into & they fly never to be seen again !. Remove the clips & the glass bed & apply a little silicone to the bed two places on each edge, place the glass in place, put the clips back on & adjust the square up & leave over nigth to cure. In the morning remove the clips. No mor flying clips ! If you need to remove the glass at some time just run a razor blade between the glass & plate.
Cheers Mike
Good luck with your hotend repairs. The Capricorn tubing will be. definite upgrade. It’s possible that the PTFE tube got swollen with the heat and that is why it is so snug getting in installed. Keep us update with your progress.