Ah, sorry, there was a line that needed to be comment out as well.
Line 4899 - this.drawItem(index + 1);
That line draws the craft button. Comment it out, as the change I had you make draws the craft button in index 4 instead of index 5.
Also, line 6217 does a check to see if you've pressed ok on an ingredient slot, or the craft button by comparing the index, you'll need to change the <= 3 on that line to a <= 2 so that it knows you're hitting the craft button that we've moved up an index slot.
Greetings. Sorry it took a bit to get back to you on this. I found a slightly more elegant solution than what I was previously suggesting.
Since I already went to the trouble of testing it out myself, I already have a patch file created. Simply load it after the crafting system plugin and it will hide the fourth slot, and skip over index 3 on the list.
It can be downloaded from the Crafting System main page, labeled as the three ingredient slot patch.
The ingredient count got replaced with the "craft button" but the help text and locked slot is still there 🤔