If you are fly sparging the dead space does not matter too much unless you get channelling as the weaker wort coming down will dilute the wort in the dead space continually and you will only get very weak wort left there anyway.
Although efficencies are nice they do not correlate to good beer, stick in some more grain if you are not getting enough length from the mash tun.
Obviously 4l dead space in a 22l tun is excessive but well done for getting it down to 2l.
