Can I convert my 4ohm speaker to 8ohms?

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.

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?