8 ohm amp to 4 ohm speaker
It is unlikely that you can extract 60W from an amplifier designed to provide 30W for more than a few milliseconds. If it were that easy you could short the amplifier outputs and extract infinite power.

You can of course use two 4\$\Omega\$ speakers in series to replace an 8\$\Omega\$ speaker, as long as you are careful to connect them in phase.