i used Test Equipment Connection about 8 years ago for upgrades to an HP ANA. they are in Lake Mary, FL

there IS such a thing as a frequency selective limiter, where it will attenuate a big signal while barely attenuating the small signal at a different frequency. they are not commercially available that i know of though.

you can add a resistive frequency equalizer that has 0 dB of loss at 4 Ghz, and 20 dB off loss at 8 ghz.
If you think of a quarter wave short circuit stub at 4 GHz, you can add a resistor between it and the main line. Since a short circuit transforms to an open circuit a quarterwave away, that resistor will not absorb power at 4 Ghz. then at 8 Ghz that same stub with the short circuited end will be half wave long, and the resistor will absorb 8 Ghz energy. You can adjust the resistor value to get the 8 Ghz attenuation you want to. You will need to use small resistors, like 0402 size.