The fourth volume of the Slayers "Choubaku Mahouden (Mega-Explosive Magical Legend)" manga, is in fact the manga version of Slayers Return, the second movie. Volume five ("The City of the Dead") continues the story from volume three ("Legend of Darkness"), which are based on the first Slayers TV series.
Why this was done - most likely because volume four was released in 1997, around when Slayers Return was released in theaters in Japan. (ADV brought it over to the US about a year ago.) That, or it was used as a sort of side-work for the author, Shoko Yoshinaka, who started work around this time on Lost Universe, a sister-series to Slayers. You'll notice that the manga started coming out in one/two-year intervals at this time, rather than her earlier three/seven-month intervals.
So, to answer your question, the Slayers Return volume that you've found online is the fourth volume you're looking for. If you're not that interested in reading up on a Lina and Naga adventure and would rather finish volumes 5-8, I wouldn't suggest buying it at this point.
Also, small note: the Slayers Special manga series and movies, while (mostly) released after Shoko Yoshinaka's manga series and the TV series, do come first in the Slayers timeline. Lina and Naga teamed up well before Lina met Gourry and the rest of the gang, so techincally, it'd be fine to pick up the Special series and Slayers Return without having finished the Choubaku Mahouden line of manga or TV series. Not sure if you were aware of this.
Might I suggest picking up the three TV series if you haven't seen it already, along with the novels? Both are even better than either manga series and the movies, and if you want the full Slayers experience, they are essential.
For more information,
Inverse.org is *the* Slayers info site.
Hope this helps!