The "standard" MkIV, either Target, Hunter or Competition models, are metal framed (Aluminum for one Target version or stainless steel for everything else) guns with the previous Ruger grip shape.
The 22/45 MkIV is a polymer framed gun with a grip shape similar to the 1911. The 22/45 MkIV is available in both "Lite", with an shrouded light weight barrel and aluminum receiver or with a steel receiver and barrel in two models.
The MkIV designation means a simplified takedown and reassembly procedure, ambidextrous safeties and some other refinements compared to earlier MkII and MkIII guns.
BTW, the 22/45 polymer framed guns were also available in earlier MkII and MkIII formats
