Because the gain of your circuit after the LNA is not high (actually you have only losses components), whatever change in NF or loss you get after the LNA the total NF of the system would not change too much.
In this situation the only major contributors to the total NF are the loss of the RX filter (one-to-one in dB to the system NF), and the NF of the LNA (degradation to 3.55dB NF to get 4dB system NF).
There is a possibility when you connect the RX filter to the LNA, you shift the gamma optimum NF matching of the LNA (degrading its NF).
Also is possible, you measured the insertion loss of the RX filter using 50 ohms at the output, but when it see the complex impedance of the input of the LNA, the filter loss to go higher, degrading in this way the system NF.
In the real life have to tune the LNA input for best system NF, with the RX filter and LNA connected together.