In fondo alla piscina

The plot becomes too convoluted during the second half, but the first 30 minutes or so forms an excellent mystery focused on Mr. Anderson's unlucky marriages to 3 women all of whom died in 'accidents' within a year after their weddings.

One of the writers was Santiago Moncada, who wrote stories and scripts for some 60 Spanish/Italian productions including the excellent neo/post/whatever noir thriller 'Corruption of Chris Miller' from about the same time as 'Fourth Victim'.

The mood is set by one of the best opening sequences I've seen in a giallo-esque picture, accompanied by Piero Umiliani's languid, dreamy theme featuring those theremin-like female vocals common to Italian 70's thrillers, ooohhh-ing over a dreamy gentle but melancholy descending melody.

We see a handsome middle aged man looking through a large window on a sunny day at a beautiful blonde woman floating alone in the backyard pool, reclining on a clear plastic inflatable chair. She looks very relaxed, slumped to one side, maybe asleep under her sunglasses, a drained cocktail glass at poolside. The dreamy sad music continues.

Then we see that the cigarette in her limp hand has burned all the way down to her fingers, but she doesn't react to the burning ashes at all. Something is wrong with her.

The sad music sound fearful and menacing as it hits an ominous chord.

The burning cig has begun to melt the arm of the plastic chair, and it gradually sinks as the inert comatose woman slowly rolls over into the water and lies floating face down. The man watching her runs down some stairs through a big mansion and out to the pool, where the woman is obviously dead....

The movie should've continued with the 'dead wives' mystery, but it goes off on a silly tangent. Nevertheless, it has a unique flavor different from most giallos, which makes it recommended.