(don't even know what the i2c pins are could you explain? ).
Forget about I2C pins.
The HC-SR04 ultrasonic sensor do not use I2C, it can use any pin you define in the setup
If that is the case, then you should be able to use the set up as described. I assumed it used A4/A5 for I2C.
As to what I2C is, it is a bus that you can hook multiple devices to. One device is the master and the others are the slaves. Most frequently, the Arduino is the master, and the slaves are sensors that the Arduino controls, but if you are having two Arduinos, one can be slave and the other master for communication. Each slave has a unique address, and you can hook up something like 112 different boards. Some i2c devices include boards that provide multiple digital pins, analog to digital conversion, digital to analog conversion, real time clock, GPS, tilt sensor, etc. Here is a tutorial: http://www.gammon.com.au/i2c-summary.