For this sort of thing I use BetterTouchTool. It's free and you can map any of a number of trackpad gestures to middle click or to dictionary lookup, including 3- and 4-finger clicks and taps.
Once you have BTT installed you'll want to turn off the system's dictionary tap. Once that's done, open BTT's preferences, hit "Trackpads" at the top, add new gesture. Choose "4 finger tap" from the "touchpad gesture" pulldown, then select "Lookup word under cursor" from the "Predefined action" pulldown (it's under the "OS X Functionality" heading within the pulldown). You should then be good to go.
I don't think BTT should cause any trouble with MiddleClick but I haven't used the latter; if there is any trouble, you can just disable MiddleClick and use BTT instead - just add a new gesture as before, mapping 3-finger tap to middle click.
In both cases you can also map 3- and 4- finger clicks in addition to or instead of the taps if you prefer those. BTT is actually pretty powerful and has a large number of gestures that can be mapped to any number of functions or key combos, on a per-app basis if desired. Of course, it's the primary reason why no one else is ever able to use my laptop :)