Can I convert my 4ohm speaker to 8ohms?

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.

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?

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.

Gerzhan - Regrettably, it isn't as simple as rewiring your woofers. The crossover/speaker relationship would now be altered. Also, since the tweet/mid driver and crossover figure into the overall impedance, the rewiring of the woofers would have only a small effect on the overall impedance. Difficult to face, but your speakers simply aren't the ideal match for a low power OTL(?). If you love the amp, try a speaker with an easier load impedance and higher sensitivity or go to larger OTL amplifier. If you love the speakers, go to a larger amplifer of the same topology or go to a more impedance forgiving, transformer coupled tube amp. Check out a high power SET (20wpc+)amp, but the sensitivity issue may still be a problem for you. Goodluck.

minimal is 3.6 Ohms, according to Stereophile. What if I rewire the woofers so that they are in series? Is that difficult?

Yes, but it would be a terrible idea. You can put a big oil bath 4 ohm power resister in series with our speaker but that would be rediculous. Check your speakers impedence curve. The Linns may be nominally 4 ohms but what is their minimum impedence along the frequency curve? Most importantly try the combination. If you don't play your music loud it might be OK. If not you will go into current limiting and things may sound hideous.