The vast majority of amplifiers will work just fine into a 4R load and are usually designed to do just that perfectly happily. Notice that nearly all amp specs will give power into both 8R and 4R loads and that most reviews will state measured power into 4R loads. It is normally cheaper, less well engineered amps that fair worse into 4R but most will probably work well enough. There are also some rather more "left field" units about which due to the topology chosen only just manage an 8R load but we are talking things like OTL valve amps here... Most conventional valve amps have taps for 4R speakers.
In general, modern amps are more tolerant of low impedance loads due to higher current output devices with larger safe operating areas being available.
Look for more than one pair of output devices per channel, a decent sized mains transformer, plenty of heatsinking and big smoothing capacitors if worried about it though.
Arkless Electronics-Engineered to be better. Tel. 01670 530674 (after 1pm)
Modded Thorens TD150, Audio Technica AT-1005 MkII, Technics EPC-300MC, Arkless Hybrid MC phono stage, Arkless passive pre, Arkless 50WPC Class A SS power amp, (or) Arkless modded Leak Stereo 20, Modded Kef Reference 105/3's
ReVox PR99, Studer B62, Ferrograph Series 7, Tandberg TCD440, Hitachi FT-5500MkI, also FT-5500MkII
Digital: Yamaha CDR-HD1500 (Digital Swiss army knife-CD recorder, player, hard drive, DAC and ADC in one), PC files via 24/96 sound card and SPDIF, modded Philips CD850, modded Philips CD104, modded DPA Little Bit DAC. Sennheiser HD580 cans with Arkless Headphone amp.
Cables- free interconnects that come with CD players, mains leads from B&Q, dead kettles etc, extension leads from Tesco