Ahh... I realize that my explanation didn't answer to the first part of your question.
But that part should be clear already from the documentation:
General discount factor for year y: r(y)=1/(1+d(y))
Present value factor for costs occurring in year y, discounted to the beginning of year z:
DISC(y,z) =∏u=z to y-1 r(u)
Therefore, you can immediately see the answer to your question "Is an investment made in 2040 discounted for 10 years by 3% and then by 4% until the baseyear or is the discount rate of 3% used for the entire timespan to the baseyear?” The answer is, of course, that the present value factor DISC(y,z) for e.g. costs occurring in y=2040 does indeed include the impact of the 3% rate for 10 (or 9) years, and the impact of the 4% rate for all the preceding years.
And the answer to your final question should be equally clear: the present value factor DISC(2035,z) for investment payments occurring in year 2035 does include the impact of the 3% rate for several years.