Note: It's best to ask your printer to specify what they have in mind. There are no stupid questions and it's better to ask at the begginign then go back and fourth with final file.
That aside - when a printed state 50% (or 25% or 10%) of original I would undesrtand that the physical dimensions should be set to that amount.
So if final print is 2x4 my file should have 1x2 (metres, inches, foots, DLs).
WHILE at the same the resolution should be upscaled by two. So if the print resolution is 100 DPI (or ppI or PPI or hoofs) then that reduced file should be 200.
Now, to specify your question. Is the 4 sheet print your requirment or is it printing process? Like for example 1 billboard is made from 6 sheets. If the splicing is done due to printing requirment then you should resize the final, whole, picture.
If the splicing is done through your requirment (like for example collages with Print on Canvas) then IMHO each sheet (frame) should be resized on it's own.
Side note - it won't be counted as 25%. It would still be the 50% of overall picture. The diference lies only in alghoritms that re-calculate the pixels, colors etc.