A 10 ohm resistor should get you in the ball park of what you are trying to do. Since I=E/R (ohms law) a 10 ohm resistor would give approximately a 480 mA load on the 4.8V pack and 600mA load on the 6V pack (the load would be slightly higher when the packs are fully charged.

If you want to get closer to 250mA load use a 20 ohm reistor.

Keep in mind when buying the resistors that they will need to be rated for the task at hand. The 6V pack with a 10 ohm resistor will require a resistor capable of handling at least 5 watts and to be safe it should be a 10 watt resistor.