speaker impedance

Lower impedance requires more current flow from the amp.  As long as the amp has the ability to drive 4 ohms, it shouldn't be an issue.  Upgrade to something that sounds good to you. 😎

If upgrading speaker. Typically you want to find speakers that captivate you… ignoring requirements. Speakers are very important to your enjoyment and if they don’t captivate you it is unlikely any amount of changes in electronics is going to help that.

While typically 8 ohm speakers are “easier to drive” but there are so many variables… I wouldn’t let it drive your choice.

If you chose planar speakers (electrostatic or ribbon), they require a lot of power (current). Then you will have reason to pause and consider what other changes you may have to do.

Best find speakers that sound great to you… then see if your electronics are up to them.

They are about the same wiht any modern solid state amp.

I mean, in an ideal world, higher is better = low current and easier to drive, but from a practical perspective, impedance only matters a lot with tube amps or speakers that dip below 4 Ohms.

The only thing really important is their SQ. In a few cases you might find a better amp to drive them. 

Buy good speakers designed well and the designer will decide which impedance works best.