I regularly selenium tone for archival permanence. I don't print on MGIV but use a variety of other FB papers including Ilford warmtone. I usually work with a 1:20 dilution at roughly 68 degrees F. The effect of toning has a great deal to do with the particular paper you are working with. Forte warm tone will quickly go rusty red if toned for more than a minute or so. Ilford warmtone will only change very slightly to rthe warm side in a period of five minutes. The dilution of the toner aswell as the particular paper will determine the color and depth of toning. The best way to do the toning is to place a print on the back of a clean tray next to the toning tray. I do this to judge the effect and depth of toning. I place the print I prints I want to tone in the solution and start a timer. It's a visual process and when you are about to the point you want pull the prints and wash again. I tone for greater depth in the shadows aswell as permanence.
You'll probably want to experiment with a few prints to see what MGIV looks like. My guess is it will cool down the color very slightly ad add depth to the shadows.