If your last ten thousand miles or so have been exclusively in the t4, you might find most or all ~9-10 oz LWTs awkwardly bulky, heavy, high-heeled, and/or inflexible. At least that's my experience, and I never got to the point of *only* running in sub 7 oz flats, yet when I slip on something like the Hayward it seems a little boot-like.
The first generation ST was, for me, kind of inflexible and unnatural - maybe too much medial post for stability. Other folks liked 'em though and not sure what the current iteration is like.
At the risk of sounding like a shill since I keep mentioning them, I love Fila's flats, the Corsa Sette (you can still find some sizes on closeout) and now Corsa Otto. They're light, pretty low profile (18/9 mm), damn near as flexible as the Free, and shockingly cushy for the weight. To me they seem nearly as protective as much bulkier LWTs - definitely a huge step up in cushioning from Nike's Streak XC, which I can't run a lot of pavement miles in. But I've not worn the t3/4/5, not sure how that compares.
Fila has another shoe slotted in between this flat and their ~10 oz LWT (Redemption) called the Volante. Haven't tried it but a few here gave it a thumbs up.
You might also have a look at the Free 3. The 5 is/was a really good shoe IMO but a little high heeled. I'm sure someone must've run in both the Free 3 and the t4/5 and can say whether the Free is more protective or not.