Can I convert my 4ohm speaker to 8ohms?

Keis is right - universal truths are rare in this field. Try them together and see what it sounds like. I run my 4 ohm speakers from the 8 ohm tap of my tube amps and the result is better than any other amp I have tried. And what is really important is I have learnt not to care that the spec's say it shouldn't be so.

Don't go messing internally with your speakers. You in effect would be changing the crossover specifications. You'd almost certainly screw up the matching, the phasing of the speakers etc. You're going to have to get new speakers or think about a new amp. Since you liked the sound of the amp what were the speaker you were listening to?

Forget about touching the speakers ! You'd ruin them. Ask the amp manufacturer whether the amp matches those speakers. Be prepared to provide info (i.e. impedance curve). Alternatively get another amp ! Amps should not be used as tone controls.