Pull-up/Pull-down resistor connected to op amp input
Both circuits do indeed have the benefit of producing a known voltage when the input is disconnected.
R1/R2 is also a voltage divider, reducing the input by a ratio of 7/(7+4.75), assuming signal source has impedance &lt;&lt; 4.75k . C1 forms a low pass, but with the Thevenin equivalent R1||R2, rather than just R1. (with the same assumption)
R3/R4 are pulling up, but R4&gt;&gt;R3, so there will be little alteration to the input voltage, and the filter is determined mostly by R3 C3 and very little by R4. (again under the assumption that input source impedance &lt;&lt; R3)