Its a bad idea to connect speakers in series because it reduces the damping factor.
The datasheet doesn't actually say that the minimum load impedance is 4Ω but this can be gleamed from the fact that: it's not specified at a lower impedance, the efficiency and THD are poorer with a 4Ω load than an 8Ω load, imagine how bad they'll be with a 2.6Ω load.
The TA2024 has a lower output power than the TDA8920 so if you bought the wrong IC if you want it to be any louder.
The advantage of Tripath being defunct is that the part is likely to hold it's value so you won't loose much if you sell it in five years time; you make even make a a small profit.