The 4-Meter (70MHz) band was originally limited to the UK, Ireland, Malta and Gibraltar, though many other countries have gained allocations recently as a result of the old OIRT FM band at 66-74MHz being dropped by most Eastern European countries in favour of the 88-108MHz allocation used in Western Europe. As a result the big names (Yaesu, Kenwood, Icom) don’t manufacture equipment for 4m,
Historically, the band was first allocated in the UK in 1956, originally 70.2–70.4MHz, but was extended to 70.025-70.7MHz in 1968, and revised again in 1987 to the current limits of 70.0-70.5MHz.
There is plenty of activity on FM, and if that’s all you want, you can buy a brand new radio like the Anytone AT-588 https://www.moonraker.eu/anytone-at-588-4m-66-88mhz-mobile-transceiver-5366 although there are plenty of ex-taxi radios for the band which will get you on the band for about 1/3 of the cost.
I use a Motorola GM350, which is available for low band VHF (suitable for 4m), high band VHF (suitable for 2m) or UHF (suitable for 70cm). There are then two variants of the GM350, one with an LED display supporting 128 channels, and one without a display and with only 4 channels. Programming new channels on the GM350 requires some freely-available DOS software, and a interface cable which can be bought on ebay. The software will work in Windows XP, but it needs a “real” RS-232 port. USB converters should be avoided, as they can “brick” the radio.
There are two of the GM350s shown below:
The top one is the low band radio and, as the display suggests, it’s on the calling channel, 70.45MHz. The middle one is the high band radio, on 145.500 and the bottom one is the 70cm DMR radio, a DM3600
If you want SSB and CW, as well as FM, the Spectrum Communications TRC4-10sL is very popular, and still available brand new, see http://www.spectrumcomms.co.uk/Amateur.htm These are nice, as they have built-in changeover switching and an attenuator, so you can simply plug in a 5W 10m radio, for example the FT-817ND, with no switching or external interfaces being required.
Microwave Modules 4m transverters occasionally appear on ebay. These are a little bit more complicated, as they have separate ports for the IF input and output, and require a much lower drive level, so unless your HF radio has a separate low-level RF out for transverters (a lot of older Yaesu radios did have this), you’ll need an attenuator and some means of switching.
Like 2m, vertical polarization is used on FM, and horizontal for SSB, CW and data. I have a Sirio vertical https://www.moonraker.eu/sirio-cx-4-68-4m-vertical and a Moonraker halo https://www.moonraker.eu/antennas/ham-amateur-radio-antennas/di-pole-halo-loop-antennas/hlp-4-70mhz-4m-halo-loop-folded-dipole-antenna
4m benefits from more frequent and stronger Sporadic E openings than does 2m in the summer, the first clue being the appearance of distorted classical music at the lower end of the band from a Ukrainian broadcast station. When you hear that, start calling CQ!
Read more about the 4m 70MHz Band on the RSGB Website here: