Well, while his personal accomplishment shouldn't be underestimated, they should be contextualized, IMO.
While he's not really the first emperor to have beaten back Umayyads, he's the first that really managed to make the best of the victories he had in face of various raids and invasions, or rebellions : the siege of Constantinople was won trough features that already existed before him, namely the strong Byzantine navy and the not-that-good Arab navy, but Constantine IV not only scored a victory there but was able to press it, and reach a balance.
That said, he was as well (but who could really do so) unable to control the various Christian banditry/rebellions/all of the aforementioned choices, in Eastern Anatolia and Syria; so he ended up reaching an agreement with the Caliphate which sanctuarized Syria from this point until the XIth.
I'm not trying to diminish his achievement there, just noticing that he was maybe more of a skilled politician than a conqueror.
As for Monothelism, I'd rather see Constantine's renounciation as a defeat of Imperial ambitions : the theological concept was pushed and supported by emperors since decades, and when Constantine IV abdicates it, he basically abdicates not only the hope to recover lost eastern provinces, but as well to appear as an alternative before Latin Christiendom (abandoning his father's ambition to resettle the imperial focus and possibly capital in Sicily, altough it was admittedly unnecessary in the 680's) and to enforce imperial position as much his predecessors wanted it.
It doesn't mean that weak to abandon an unenforcable policy, but it clearly set a new situation where the Byzantines accept to be be an Anatolian-Balkanic empire rather than universal IMO.
So what do we have there? A skilled and realistic ruler, in a new and still troubled by raids and usurpation, period of Byzantine history. To be honest, I'm not really seeing him being that adverse to the same policies Justinian II pulled historially, except that his policies may have been less ambitious than his son's for what matter the restablishment of imperial authority in eastern Anatolia and in Constantinople itself. I'm not really sure he would have done that better in face of caliphal reinforcerment, but better enough could be enough to fend off most of the opposition to imperial authority (that Constantine IV, I'd rather think, participated to mingle just a bit).