BNOS of Essex once produced a wide range of power supplies and linear amplifiers for the radio amateur, and will now still make linear amplifiers to order. Their products are also often seen on the second-hand market: look out for the LPM 70/10/100. This produces a true 100W output for 10W input, includes a receive preamplifier as well as a LED power output bargraph and SWR protection. These units are worth hunting out if you are looking for a 100W amplifier for the 4m band.
An ex-PMR linear amplifier which is ideal for the 4m band was manufactured by Pye of Cambridge. The A200 linear amplifier was designed to boost the output of AM mobile transceivers to about 50W PEP output. The E0 model (marked on the serial number plate) is designed to operate from 68 - 88 MHz and will tune to the 4m band without any modification. They are designed to operate from 13.8V and include RF VOX and RF changeover relays. The only modification which may be desirable is to provide a facility to hard-switch the amplifier to prevent the relays chattering in SSB operation. These units can be recognised by their black cast housing with a cabled input connector and chassis-mounted output connector. Don't be deceived the three core 'brown, blue, green' power cable - these amplifiers are for 13.8V DC operation. Again these units have been widely seen on surplus stands at radio rallies at bargain prices, particularly for the E0 versions.