I don’t think there’s anything wrong. You can choose to consider it as a regular annuity if you’re sensitive to the adjustments you need to make (a timeline usually helps with this). This is true because a payment at a beginning of year X is, for all practical purposes, a payment at the very end of year X-1. With this in mind, here are two ways to arrive at the same answer:
>>regular annuity (calculator in END mode all the time):
we have 4 payments of 20k each starting 18 years from now (ie. at the beginning of year 18). The PV of those is 70,919. Notice that this is the PV at the beginning of Year 17 (since it’s a regular annuity). Now this is the FV of the regular annuity with 17 payments. Set i=0.05, n=17, PV=0, and PMT should get you the right answer.
>>annuity due (calculator in BGN then END):
Start with BGN mode. Calculate the PV of an annuity due with 4 payments of 20k at 5%. That should give you 74,465. Notice that this is the PV at the beginning of year 18 (=end of year 17). We need to discount it at 5% to bring it to the PV at the beginning of year 17; that is, again, 70,919. This is our FV for the regular annuity. Switch back to END mode and repeat from above.
Notice that in both cases we choose the beginning of year 17 as our FV. This is because, for a regular annuity, that’s when the 17th payment is acutally made. Try to visualize the timeline: payment 1 is made at the end of year0, p2 at year1_end… p17 at year16_end (or equivalently, beginning of year 17).
Hopefully this made sense.