From your description, "K" stem soft wheel casters appear to be your best bet. However, I can not make this recommendation without reservation because of the unusual shape of your caster stems. I recommend contacting Tempur-Pedic® for more information, or emailing us a picture of one of your casters. We are happy to do all we can to find the right casters for your chair. Just be sure to include the diameter of the narrow part of the stem.
Master Caster® casters are sold in sets of four or five. The "K" stem Master Caster® Duet Twin Wheels are sold in sets of five. The listed prices are per set, not per wheel.