Ah yes, thanks for the tip
So d{B}/dt = k1{A} - k2{B} - k3{B} - k4{B}{H+} = 0
Rearranging gives k1{A} = {B}(k2 + k3 + k4{H+})
Therefore {B} = k1{A}/(k2 + k3 + k4{H+})
Since kobs = d{C}/dt = k3{B} + K4{B}{H+} = {B}(k3 + k4{H+})
Subsittuting for {B} gives kobs = k1{A}(k3 + k4{H+})/(k2 + k3 + k4{H+})
Which looks sort of correct? Sorry for the messy formatting
However there is still the [A] term, I was wondering if it was relevant as the concentration used remained constant throughout the experiment.