I don't think you could classify 4ohms as particularly difficult. The 1070 has a 4/8 switch and I suspect you get closer to 100wpc with 4 ohms. My speakers are 6 ohm and the 1070 is set to 8 but I haven't had a chance to drive them into nosebleed levels... yet. Some of it would depend on offloading the low frequency signals to a sub and then setting the Xovers appropriately. If set the main speakers to cross over at 80Hz or something most of the 1070 amplifier power would be driving upper frequencies and wouldn't need the stump pulling power that pumping out 35Hz would require.
I think my Wharfdales are only 93db efficient and in my 12 x 20 listening room it seems to get pretty loud although I haven't got my db meter yet... ;-)
1070, Arcam CD73T, Pioneer DV46-AV, Project Xpression-Grado Red-NAD PP2, Monster HT1600, Wharfedale 8.3 Diamond, Mirage OM200 sub, Omnisat Micros x 5, homemade speaker cables, Dell Axim 51v remote