The part number for the .380 taper crimp die is 7153100.
You can order direct from Lyman (at full retail price...spendy), or it's available at midway too. Probably other online reloading supply houses too.
If you've got the Multi-Deluxe 3-die set (part #7680112), then you should have the following:
1 carbide sizing/decapping die
1 expander die
1 seating die (probably with two or three seating stems for the die...one for round nose bullets, one for flat nose style bullets, and sometimes one for hollow points).
According to the Lyman catalog that I've got (2008) and their site, they don't offer a 4-die set for .380ACP.
The 4-die sets include the three dies listed above and also the taper crimp die. 4-die sets aren't available in every caliber.