Often enough clients do not know themselves what to request. 2 weeks later there will be that call "At which resolution can we get the images? Because now we decided to print a flyer".
So I would at least do something printable at A4 to A3 (3.2 to 3.6k or 4k as mentioned). For a web presentation they may want to use some crops depending on page design anyway.
Do no 1080, do 4k but deliver web resolution. If they later ask for a higher resolution, charge them for the hires version. For you this should only mean longer rendertime and a few MB more harddisk space in the first place, but no additional work.
Good Luck