My personal recommendation, if you like the first, is to at least read the next two (Dune Messiah and Children of Dune). If you're still liking the series, finish the next three (the tone does change with God Emperor and again with the last two).
It should be noted, the first three books form a trilogy. The fourth book is a bridge to what was intended to be a second trilogy. Herbert finished the first two in it (books five and six). His son (with Kevin J. Anderson) wrote several prequel trilogies (I did not enjoy, YMMV, read at least one of them if you've made it this far to determine if you'll continue, quick reads), and eventually concluded the original series with two additional novels (I haven't read them, I was turned off by the style of the first prequel trilogy and haven't returned to it).
It should be noted, the first three books form a trilogy. The fourth book is a bridge to what was intended to be a second trilogy. Herbert finished the first two in it (books five and six). His son (with Kevin J. Anderson) wrote several prequel trilogies (I did not enjoy, YMMV, read at least one of them if you've made it this far to determine if you'll continue, quick reads), and eventually concluded the original series with two additional novels (I haven't read them, I was turned off by the style of the first prequel trilogy and haven't returned to it).