"how to apply the "hold on" to each figure"
The properties affected by hold are all axes properties, which have nothing to do with figures. When you read the hold documentation it optionally allows an axes handle to be specified (not a figure handle, which hold has nothing to do with).
It is trivial to use a loop and indexing to access graphics handles, e.g.:
for k = 1:4
fh(k) = figure(k);
ax(k) = axes('parent',fh(k));
...
hold(ax(k),'on')
...
end
