Arduino output pin from 5V to 3.3V
Your code does not configured SignalPin as an output, but only trigPin.

As already explained in comments, your observed voltage is due not really to the voltage divider you constructed, but rather one formed by the "weak" internal pull-up resistance and your external resistors.  From your measurement, it appears the value of the weak internal pull-up is about 35K, which is within the range of expectations.  That and your 4K external form the top of the divider (39K) while your 10K forms the lower half, yielding about a fifth of the supply voltage.

In some versions of the Arduino library, writing to an input pin enables and disables the internal pull-up resistor, so you could have observed your program seeming to control the pin - but at minimal drive strength.

To get the desired behavior, you merely need to add

pinMode(SignalPin, OUTPUT);