What would be the resistance? 3 ohms?
When resistance is in series, the values are added. Impedance is defined as "resistance to AC currant", which means that it will vary, according to the frequency. The spec sheet for the speaker should indicate the minimum impedance and it may drop to about 3 Ohms, but that shouldn't be a problem for the amplifier since the original 4 Ohm driver would also drop to about the same impedance.
If you wire the voice coils in parallel, you would use (Z1 x Z2)/(Z1 + Z2). Z is the symbol for impedance. If you need to wire more speakers or resistors in parallel (in this example, it would be three components), you would use the formula 1/(1/Z1 + 1/Z2 + 1/Z3). This works for any value of resistance, so if you have a 4, 8 and 16 Ohm speaker, you would find the least common denominator and then add the top number and once you find the total, you invert the fraction. 16 is the denominator, so 1/4=4/16, 1/8=2/16, etc so 1/(4/16 + 2/16 + 1/16) = 1/(7/16), which is 16/7, or 2.28 Ohms.
