creating a formative construct from 4 items in SPSS



Below is an excerpt directly from a paper that I am attempting to replicate. The 4 items are survey data collected on a likert scale and I have a sample of 150 responses. I would appreciate any help as to how I would go about using the 4 items to create a single construct as they describe below:. How do i go about doing this in SPSS.

"We developed the construct target flexibility out of four
items adapted from Van der Stede (2001). We treat the
construct as a formative index across the four items
(Bisbe et al., 2007; Diamantopoulos & Winkelhofer,
2001). For this formative index-based construct, observed
variables cover different facets of the construct and should
not have significant intercorrelations. The individual items
reflect the extent to which targets are continuously monitored
and potentially adjusted. They also reflect when
underlying assumptions are no longer valid. A reversecoded
item captures firms’ unwillingness to change target
levels at all intra-year"