How about 10 SPD DA for 11 SPD 105 ?
Does that mean that you've got 10 speed DA then? If so, why not just change the chain and cassette for Tiagra when they wear out? A change to 4700 Tiagra will mean that you'll also have to swap out the rear derailleur as well, as the pull ratio of the new 4700 stuff is different to your current rear mech.
I suppose if you've got the old 7800 DA shifters, the ones with the cable coming out the side, it might make sense to change them for new 4700 shifters as they've got the cable under the handlebar tape routing. You could probably sell the old DA shifters on
EBay and recoup at least some of the cost... whether that would be worthwhile or not depends on which one you prefer the feel of. Perhaps it'd be worthwhile finding someone with a 4700 groupset and trying it out to see if it's any better than what you've got?