A 4" Senko is appears to be .375". There is a skinny carrot that is .3125 and the 4" is .375. Here is a picture of the 4" senko and carrot:

While the action is very similar, you can see there is a difference in appearance.
The 3" skinny carrot is an inch shorter and .3125 vs. .375. Sorry I forgot to add it to the pic...
If you're not sure just order a sample bag and try them out.
Tight lines!
Jason