The Type IV or metal-formulated Compact Cassette was introduced in 1979 by 3M under the trade name Metafine, and addressed a number of problems with previous tape formulations.
Standard Compact Cassettes (Type I or ferric-oxide) typically had poor high frequency definition, with pervasive tape noise (hiss). Type II (chromium dioxide) had much better frequency reproduction and very low noise but at the expense of some output level and low frequency solidity. Type III (ferro-chrome) was an attempt to restore some low frequency firmness but whilst Type III worked as a concept, it was not enough of an improvement to be taken up by the market.
Type IV was a completely new formulation using pure metal particles instead of metal oxides. This created a hard-wearing tape with superior frequency response and greater dynamic range.
It also had some disadvantages such as excess wear on tape heads, and they were expensive to buy. By the mid-1980s, metal tape did begin to catch on and was adopted by a lot of enthusiasts.
For tape recording equipment that was capable of using metal tape, there were extra indentations in the top of the cassette to identify it as Type IV and select the correct bias and equalization. On equipment without metal tape settings, it was possible to playback cassettes, but recordings would not be successful.