Apart from good wages, which benefits would you use to attract the best developers in town to join your small team?
What kind of office appliances/toys/furniture would you invest on?
Other than that: ergonomics. Don't skimp on the chairs and keyboards. Big monitors are good but an aeron chair shows that you know what's up and give a damn. YMMV.
It's expensive to hire someone, its expensive to manage someone, it's expensive to keep someone informed about a project through meetings/documentation etc, it's expensive to rent office space for someone, it's expensive to have to hire more people to get the same productivity.
And all of these expenses are weighed against the reduced productivity of those extra two hours, and at 99% of companies they've decided it's better to hire for 40 hours.
The studies may be wrong, or maybe you pencil it out and it just doesn't model out correctly, but if you lock in the pay on the basis of the assumption that the total productivity would be less, it's sort of favors one side of interpretation. And if you're truly trying use a four day week to attract high-end talent, reduced per hour pay somewhat dilutes the attraction of the benefit.
Finally, some consulting gigs are billed per hour anyway, so the model might end up that total comp is somewhat reduced vs a full 40/hr week. Or it might end up the same pay for four days - it really depends on the business model.