There are several tube amps that can handle a four ohm load. I have a great deal of experience with the Wolcott Presence mono blocks, and assure you they will produce in excess of 200 watts per channel with no clipping, even at loads less than four ohms.
Atma-Sphere MA-2 tube OTL amplifiers have no trouble driving a 4 ohm load. I drive Avalon Eidolons, a nominal 4 ohm load, with this amplifier with great results. Generally, however, tube mplifiers will be happier with higher impedance speakers.
I have Aranov tubes on my Kef 4 speakers that are 4 Ohm. The Aranovs are noted for a good bottom end ....but I have a REL subwoofer also to help down low. Other than the Aranovs, many tube amps do have a specific setting for 4 ohms that will help match the load.
More important than the actual 4 ohm rating is the impeadence variance (i.e does the speaker drop to 2 ohms or even lower). Impeadence variance below 4 ohms can make things difficult for even the best amps. Efficency and design of speakers, and of course, power of the tube amp, also factor into the equation. -Lorne
David Berning ZH-270 otl definitely stable into 4 ohms, no problem. I am currently using them to drive Soundlab Pristine ll's with outstanding performance, the 270 can drive then up into the mid 90db's without strain and they drop down to at least 3 ohms. This is the only OTL on the planet that is able to actually increase its output power steadily as impedance drops. 70W@ 8 ohms (typical output power at onset of clipping 84W@ 8 ohms, 110W @ 4 ohm). This amp has the MOST drive and musicality of any amplifer I have ever heard. It also incredible bass output, unlike other OTL's.
I HAVE A 30WPC (TRIODE/KT88) AUDIO ELEC SUPP (CARY) SUPERAMP SIGNATURE EDITION. IT IS CURRENTLY DRIVING 2 WAY STAND MOUNTED MONITORS FROM THIEL (NOTORIOUSLY HARD TO DRIVE) THEY ARE 4 OHM & DO DIP BELOW THAT. IT IS AN OK COMBO, BUT I HAVE A SMALL ROOM, & CAN'T PLAY AT CONCERT LEVELS IF I WANT TO. I WOULD SWITCH TO A LARGER, 8 OHM 3-WAY THAT IS EASIER ON TUBES, IF I CAN FIND SOMETHING THAT IS AS DETAILED & ACCURATE AS THE THIEL SCS3'S.
The Kora Cosmos amplifiers are rated 100 watts class A triode from 2-12 ohm loads. Since I am a dealer I won't give you my site but Kora's direct. It is www.kora.net. We have driven speakers that dip as low as 1.5 ohms with these amps with no problems.
Hybrid amps may be another safe bet. I'm using a pair of Blue Circle BC2s (hybrid - 75wpc Class A, with one 6SN7 tube per amp) to drive my Audio Physic Virgos, and they seem to do a wonderful job. I believe the CEO of Blue Circle uses the same amps to drive a pair of Martin Logans (can't remember the model) that are notoriously difficult to drive. Cheers, John.