Hi Tom,
yes, that is correct. The device can only measure three single ended signals as you need to sacrifice one input as the common/negative input for the other three channels.
In case the sine wave swings around the DC offset of 1.25V but has a max. amplitude that is less than 1.15V, then you should be able to make it work with ADS1147. You would just have to connect the 1.25V to the input that you use as the common input. The positive input could then swing from 0.1V to 2.4V.
If you need samples or an EVM for the ADS1120 please let me know as they are not yet orderable on the web.
Regards,