Hi @Abdulla Anees,
You can create multiple artboards side by side to section your work. Next, draw a rectangle over the first artboard/section and create a clipping mask. This will hide everything else on the other artboards. Delete the other artboards and save this first section as a pdf (to make a smaller file, open the file later and remove the excess images). Undo the deletion of the other artboards and release your clipping mask. Create a clipping mask over your second section. Delete all other artboards, and save this second artboard (opening later, again, to remove excess images to decrease file size). Continue to do the same steps with the other 3 sections.
As long as you have your artboards butted up, you shouldn't have any issue with fluidity. The only thing you'll need to find out though, is if the printer requires bleed / if there needs to be any overlap. If so, you'll need to adjust each clipping mask and artboard BEFORE deleting anything from each individually saved press file.
Also, depending on how far back people will be viewing the billboard, you can lower the dpi quite a bit. For example, for a massive billboard that you view from a highway, you can go as low as 72 dpi if needed. Keep the dpi up as high as your file size and printer will allow, though.