Hello DMN,
Seanbrown has already remarked that +-4.5V is maybe too low. The figures in the datasheet show voltages above +-5V, which you should try.
E.g: Figure 12. starts at a power supply of 10V and fig. 5. gives us the hint that the on-resistance increases as you decrease the supply voltage.
Is the output of your multiplexed channel loaded with a low resistance? Maybe Ron is already in the range of some thousand ohms.
Regards,
