There are several 1-4X24 AR scopes on the market. Nikon has the M223 and the lower cost P223 scopes. Burris has a nice one. Vortex has two, the PST around $500 and the Razor HD, around1.4K.
How much do you want to spend? I wanted a Razor but could afford a M223. After two years with the M223 I wish I had saved for the Vortex PST, worth the extra money IMHO.