Not sure about you but i don't know how to use regex properly so I made a longer roundabout way of doing this using just what's available under the scripting functions on the main website.
Artist 1, Artist 2, Artist 3, Artist 4
In the example above i've made a formula that would change the last comma to the & sign.
$IF($AND($NOT($GRTR($strstr(%Artist%'&'),0)),$NOT($GRTR($strstr(%Artist%'Ft.'),0))),$cutright(%Artist%,$SUB($ADD(1,$Len(%Artist%)),$strrchr(%Artist%,','))) $Replace($cutleft(%Artist%,$SUB($strrchr(%Artist%,','),1)),',','&'),%Artist%)
So i started with what my rules were firstly the Artist field can't already have an & or Ft. because I'd probably risk duplicating the & or having a misplaced one.
These rules state that the "String" & as well as Ft. are not present in the Artist Field
It basically returns the character number for the first occurrence of that string in the field specified, if it's never found the returned number is 0 which is why the (grtr) greater function is also used here. If the first occurrence is not greater than 0 then it must not exist within the string "Artist"
$NOT($GRTR($strstr(%Artist%'&'),0))
$NOT($GRTR($strstr(%Artist%'Ft.'),0))
Next I have some variables I will need to do some simple math
$strrchr(%Artist%,',') - Where is the last comma
$Len(%Artist%) - count the character length of the whole artist field
I then used the cutleft & cutright in conjunction with some add / subtract to do a replacement within a subset of the Artist field. I basically use the cutleft / right to isolate part of the artist & then I can replace just one of the commas instead of all of them.
If I know where the last comma is & I get rid of everything in front of the last comma that means all thats left is ", Artist 4" then I replace the comma from this substring & join it wilth its opposite substring of the original artist which would be "Artist 1, Artist 2, Artist 3" Note I added a space between the two in the formula.
I hope I have explained this well enough to follow. Whenever I come across issues I have to use my lack of regex knowledge to come up with something creatively convoluted.