First, you will have an impossible time of trying to have the 4" end of a 6x4 R/I fitting go oval at the stove. At the cast stove oval is the only point in the pipe run where the rule is broken and the pipe goes over the oval. It's nice also to caulk that joint from inside AFTER the pipe is installed. What I'd do is put the R/I at the "down end" of the insulated pipe run and then use 4" single wall pipe from that point down to the stove. Even if you could make happen what you propose, to my eye, 6" S.W. pipe on top of a small stove looks odd/top heavy. This is the common way of transitioning between 6>4 if one wants to use 6" insulated pipe. A/NSW