First of all, great mod. It solves a lot of issues I had with my pipe layout.

However, there are some things that could be improved. For example it would be nice if there is an icon on the pipe, indicating what fluid is in them, like in most of the other pipe mods.

Also there is a minor bug, where the recipe shows an error, Unknown key: "item-name.pipe-to-ground 3" for the 4 way and "item-name.pipe-to-ground 3" for the 3 way, instead of the actual name. The names in the player inventory and the world are correct, though not very descriptive. A better name would be advised.

Again, other than that, it's a great mod!

Thank you.
I fixed the 'unknown-name' thing for the next update.

About the naming:
Technically you're correct, it is a 3/4 way cross.
The reasons i named them pipe-2/3 are the overground connections and the possibility to do that without using custom translations so this mod works for all languages.

Icons:
There are none because vanilla underground pipes don't show one either. After all this is just a pipe.
In theory it could be possible to use a furnace type with a custom category and add a pass-through recipe for every fluid. This would remove the ability to replace this 'pipe' with other ones so you have to mine it first.
Would adding a window like the normal pipes have kind of solve this problem?

Yeah, naming is always a pain if there are translations to maintain. So, it is understandable that you decided on a completely neutral naming scheme.

In vanilla, the 3/4 way shows the icon in the "straight" section (https://imgur.com/a/0XtjocG). These pipes are pretty much that straight section, it feels a bit off if the icon is not there.

And as far as the replace function goes, these pipes can only replace down-pipes, but I think most people would replace the straight and then delete the down-pipe. At least that is how I use the mod.