To reduce the original current by half is only possible by doubling the total resistance of the circuit.
Setting the restance of each resistor to 1 unit, the total resistance is at the beginning equal to 2 units.
To reach 4 units as resistance for the complete circuit, R2 has to be increased by a factor of 3. This will add 3 units to R1, resulting in a total of 4 units for the complete circuit.

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