You cannot use two different markers on a single line, so your best bet is two overlapping lines, and you can use MarkerIndices to control which vertices on your line use markers. For example:
R = 1000:1000:10000;
I = 0.0001*ones(size(R));
V = [4.901, 4.802, 4.703, ...
4.604, 4.505, 4.4059, ...
4.3069, 4.2079, 4.1089, 4.0099];
tbl = table(R,I,V);
% Plot the same line twice
p = plot(tbl.R,tbl.I,tbl.R,tbl.I);
% The first line will have a solid line style, dot markers, and use
% MarkerIndices to control which vertices have markers.
p(1).LineStyle = '-';
p(1).Marker = '.';
p(1).MarkerIndices = find(tbl.V>4.5);
% The second line will have no line style, * markers, and use
% MarkerIndices to control which vertices have markers.
p(2).Color = p(1).Color;
p(2).LineStyle = 'none';
p(2).Marker = '*';
p(2).MarkerIndices = find(tbl.V<=4.5);
