I would recommend to get an enlarger with a soft light and a head for BW printing, that allows you to dial in your contrast setting, such as a Durst L1200 with a variable contrast head (VLS 501). Alternatively with a color head, which will let you do the same, but not quite as straight forward as a dedicated BW head. I would stay away from hard lights, because they emphasize any imperfections in (like scratches) or on (dust) the negs. For 4x5 you will definitely want a glass carrier.
For 4x5 you obviously need a 150mm lens, which you can use for 6x6 as well, unless you enlarge to a high degree or find your printing times get to be too long. Then you would look for an 80mm lens. Any Schneider or Rodenstock lens would be great.