C4 and C5 lowers the impedance in emitter thus increasing the transistor's stage gain with frequency (the higher the frequency the lower impedance of the capacitor C).
Without the capacitor the stage gain will be reduced and given by the ratio of collector vs. emitter resistor values.
The higher the frequency the lower the emitter impedance with the capacitor (and the higher the stage gain).
In your case the impedance in emitter will go from 330ohm (stage gain aprox 4) at low frequencies (around DC) to 30ohm (stage gain aprox 40, 330ohm parallel with 33ohm) with high frequencies.
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