Before.

Taxing a redistributive payment from the government makes no sense. Why not just pre-calculate how much of it goes to tax and then save everyone the paperwork?

I suspect it is exactly to prevent paperwork. Having two different tax-systems, depending on your employment status is going to be a huge mess.

A basic income does not depend on your employment status. Your point stands, however - treating every dollar the same can be simpler.

4000CHF was regarding the minimum salary, not basic income.

Alexandre and Christoph both earn the same amount of money. Alexandre has no employment expenses. Christoph has 1000 SF per year employment expenses. Income is defined as revenue minus expenses. Thus Alexandre will pay more tax than Christoph, because Alexandre has a higher income.