flooding 4 layer PCB
Generally I try to flood as much of the board as possible to ensure consistent heating over the entire board during the reflow process. Leaving large holes in the copper pours can cause some parts of the board to heat up more than others, potentially leading to warping. Additionally, large holes in the pours potentially means a longer time to etch, which may cause over-etching in the smaller areas (around traces). I recommend flooding as much as you can, even if it's just dead copper.