I think that for tax-loss harvesting, the appropriate question is, "if A is my preferred choice, and B, C, and D are my substitutes... during the length of time I plan to hold B, C, or D, how much difference is there likely to be between holding them and holding A?" Notice, by the way, that it's quite possible that your substitute could outperform your preferred choice; in fact, if they are pretty similar, the chances are about 50/50.
I'm going to do a quick reality check and I will use "the last full calendar year," i.e. 2016, as the "holding time." Before I look, I will hazard a guess. Since the first three are all indexes that include developed and emerging markets, while the third is developed only, I would expect to see more difference between VTMGX and the other three than within the first three. On the other hand, since developed markets are, after all, about 81 to 87% of the total market (depending on whose definition you use!) I would expect the difference to be small. OK, here goes.
The three funds that include both developed and emerging markets are in blue, orange, and green, and the one that includes developed markets only is in red.
My personal conclusion is that during 2016, VTIAX, IXUS, and VFWAX looked almost like clones, and VTMGX didn't. Once again, there's no reason to assume that VTMGX will always underperform, but if your goal is to stay close to VTIAX, then--as you'd expect simply from their index descriptions--IXUS and VFWAX are closer to that goal.
Everything I've ever read about tax-loss harvesting and wash sales says that nobody really knows
what the IRS will treat as a wash sale--that is, the IRS has never defined "substantially identical"--and I've never read of the IRS objecting to tax-loss harvesting between similar mutual funds or ETFs. I imagine they would give some warning if they decided to. That said, personally, since VTIAX and IXUS include some
small-caps, and VFWAX includes no
small-caps, it might be worth noting that with VTIAX/IXUS or VTIAX/VFWAX it would be easy to point to a very specific difference between them, not only in the actual holdings but in the stated definition of the intentions of the indexes they track.