HD3 and HD4 are the diamond pixel ones that use "wobbulation". IIRC they use half the horizontal resolution and essentially interlace the image (so for each horizontal line, one pixel on the chip is used to light two pixels on the line). A cost effective way of displaying a high resolution image, but softness issues are true especially as the image needs to be reprocessed to split onto the chip. I avoid these the same way I do ALIS plasma screens (which do a similar thing). However they are much cheaper to produce, I can't see the 1080p wobbler being too much more expensive than 720p non-wobbler. So prepare for an in-flux of 1080p DLPs and hope that the latest version of the wobbling technology isn't as soft as it has been!!
Now I hate the TI codes as much as everyone else does, mainly because they keep changing and don't seem to follow an order!!! I'll give it a go though:
HD2 is 720p with proper pixels, HD2+ then used early dark metal technology, closer mirrors, and I think a different angle of tilt all basically combined to give better brightness and contrast.
DC3 is Dark Chip 3 but this is a technology not a specific chip model number. So I see it as you can have a normal HD2+, or a HD2+ with DC3 in it. This is the newest Dark Metal, is awesome, and is filtering down now to all chip sizes.
HD3 was the wobbulated 720p (640 x 720?)
xHD3 is wobbulated 1080p (960x1080).
These then changed to HD4 and xHD4 but still mean the same resolutions. The 4's having faster switching speed for smoother interlacing of the image.
I wonder if HD4 is still otherwise the same as HD3, or if some Dark Metal and other technologies have made there way in too. The HD3 is quite an old chip now, outperformed by the HD2+ without the latest DC3! Although I do know the xHD3 (1080p) launched last year did have DC2 technology in it. I've only ever heard of HD3/HD4 being in RPTVs (and haven't seen a HD4 set yet - hopefully one at CEDIA this week), not sure if it will go over to front projection or not.
What I couldn't tell you is if the HD4 and xHD4 still suffer the softness issues as these are very new out. The xHD4 is likely to appear on some high end single-chip DLPs before the end of this year, and I don't think it's going to be significantly more expensive. So on the surface 1080p for not more than 720p money, but behind closed doors using a different technology that traditionally hasn't been as good.
There will eventually be a 1920x1080p but God knows when and I am sure it will be very expensive. This might be what the HD5 is?