The amplifier can provide a max voltage (the output is specified as power in a specific load) in the output (for a specified distortion),
when you apply this voltage to a 8ohm load then you will have a current , for example 8v will result in 1A and 8v*1A=8w
when you use a load with half the resistance as 4ohm then for 8v it will try to get 2A (8v/4ohm) so 8v*2A=16W.
The only problem is that amplifiers are not usually able to double the power in 4 ohm loads because they are not able to provide double the current with low distortion (there are many exceptions that can double or quadruple (4x) the output down to 2ohm or 1ohm but these are usually expensive amplifiers with high current psu and output stage).
Your amplifier probably has a specification written in the manual so check to see
if it can work with loads lower than 8ohm and how many watts you can get.
Alex