You should ask an actual modder with current knowlege, as I just tinkered (over 10 years ago) but consider getting to know the 2da files, under (I think) Rang01 to Rang04 - each number was a kit, then locate the special ability ABSPCL301 (example) which designates the backstab modifier and either delete the next one (that you'd get at a higher level bringing you to x4) or ask someone if you can create a new ability (as above but ensure it's unique) to reduce it by 1.
It may sound more complicated than it is, Near Infinity was the tool I used, I'm sure there's another one now.
This is all assuming it actually raises to x4 as you level up anyway.
I think more likely the stalker used to have an ability at level 1 and 9 that granted it +1 backstab multiplier; when it was externalized, it became tied to the 2da and the level 1 / 9 ability was removed, but any character that had already been created would still have that ability, resulting in a net bonus.
That's pure speculation, though; I haven't looked specifically at the Stalker's CLAB table to see what it used to be.
I can confirm that this anomaly only occurs with pre-patch saves being loaded after the patch.
Its not the end of the world for me. My stalkers only one level away from getting the 3x backstab multiplier anyway (level 9 I believe). My only concern is that if I import this character in to bg2ee will the multiplier change to 4x once I reach the correct level (I think its 17).
You can fairly easily edit your character with Near Infinity or EEKeeper to fix this issue. Or BG2EE might even fix it by removing the incorrect bonuses as I suggested.
Comments
It may sound more complicated than it is, Near Infinity was the tool I used, I'm sure there's another one now.
This is all assuming it actually raises to x4 as you level up anyway.
That's pure speculation, though; I haven't looked specifically at the Stalker's CLAB table to see what it used to be.
I can confirm that this anomaly only occurs with pre-patch saves being loaded after the patch.
It's not as if all characters start on a level playing field anyway.
Its a new game with my stalker exported from my pre 1.2 patch.