Kinda off topic, i know you're generally not supposed to mix brands or sizes unless ya really really know what youre doing and/or got some intense staging n what not going on.

But can you mix a 4ohm sub with an 8ohm sub?

ya you can mix the two. but the 4 ohm sub will get more power then the 8 ohm sub. in parallel the load will be 2.67ohms and in series 12ohms. so the 8ohm sub will get 1/3 the power and the 4ohm will get 2/3's.

In parallel the 4 ohm draws twice as much current then the 8 ohm. 4 ohm consumes twice the power of the 8 ohm

In series the 8ohm drops twice the amount of voltage than the 4 ohm. 8 ohm consumes twice the power of the 4ohm.

parallel connections deal with current draw, series deal with voltage drop.

so generally the answer is no lol