itch.io

You have to use wadsmoosh to compile all the IWADs together and use E1M4b and E1M8b as mods.

This is an old post but just in case anybody else winds up here:

Loading e1m4b.wad and e1m8b.wad separately (as the reply comment below this one says) will replace E1M4 and E1M8 with the new versions, and you should do that if that's what you want.  If you do this, the two WAD files aren't specific to this mod and can be downloaded from any of the usual places.

If, on the other hand, you prefer to play the original E1M4 followed by the new one, and the original E1M8 followed by the new one (so the episode now contains 11 maps total if you count the secret level), then our fearless leader here has created a special WAD to provide that behaviour; this link is to another comment on this same page with the download link:

https://itch.io/post/4237949