Sure, you can install a propane stove top,but an induction cooktop would have fewer indoor air quality issues.
But why would you use 4" of closed cell foam? The reasons I ask is...
That's less than R30, and thus won't meet code min anywhere in the US. (R30 is for zone 1 southern FL or Hawaii, everywhere else needs R38 to R49)
When placed between rafters, the thermal bridging of the rafters rob it of a significant fraction of it's performance due the R5-ish R value of the framing. At a 15% framing fraction the average or "whole assembly" R value of a roof deck sprayed with 4" of R7/inch (R28 center cavity) foam comes in at about R18 (U0.0.55) using GENEROUS assumptions for the R value of the roof decking and the interior & exterior air films, etc.
Most closed cell foam is blown with HFC245fa (with the exception of LaPolla's HFO1234 blowing and Aloha Energy's H2O blowing agent). This is a powerful greenhouse gas (about 1000x CO2 @ 100 years.)
At 4" closed cell foam is extremely vapor tight at 0.15-0.25 perms, offering extremely low drying capacity for the roof deck.
Closed cell foam is very expensive, at about 18 cents per R per square foot.
There are cheaper and lower impact ways to get there (or even to code-min!), but the moisture safe particulars vary by climate. Where are you located?
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