pick a current and use R=V/I.
if each LED needs 15mA of current, and has a forward operating voltage of 1 volt, and you have 4 in parallel, then you need 60mA of current at 1 volt. with an 9v battery, you would have R=(9v-1v)/(.06A) = 133 ohms.
for quickly estimating resistors, as a general rule of thumb, i use at least 330 ohms for one LED at 5 volts. doubling the voltage means doubling the resistance, and adding more LEDs in parallel means halving the resistance. so for 9v you'd about double the resistance to 660, then divide by 4 for the number of LEDs and get about 165 ohms.
either way, most LEDs will light just fine on 10mA or less, so you should be fine using a resistor somewhere around 200 ohms.