There is a lot more being shown in HL2 then in Resident Evil 4, it has higher res textures, AA, much higher resolution (I'm pretty sure RE4 doesn't have HD compatibility), HDR (loast coast), physics and ragdoll instead of scripted deaths that RE4 has. You can't compare a computer game to last gen console.
RE4 is a great looking console game for sure but it just can't stand next to computer graphics.