1) What exactly is a "good track"? A "fast track"? "Slower track"? "Muddy track"? I've assumed fast would probably be the standard dirt races. Muddy I guess are when it rains. Not sure on good and slower. (Is turf slower or faster than dirt?)

2) This might be answered in #1... but with that "going preference" can I tell if my horse is going to like running on dirt or turf? How? Or do I just have to run them on each and see how they do.

Some horses seem to run better on one or the other, some run just as well on one or the other. But is there a better way to find this out then just experimenting?

3) When breeding, does the stallion or the mare make the bigger difference in the quality of the offspring? Or is it about the same?

4) When selling *unraced* 2 yo's, I have found a wide variety in how much they sell for in auction. There was one horse I spent 12k on to breed and sold the unraced 2 yo for 274k. But then I have the opposite thing happen alot too. I spent 52k on one and only got 28k back. So, what do the trainers look for in the auction? Group 1 winning parents?

The track conditions relate to how firm or soft the horse likes the track conditions. Ranging from firm, fast, good, slower, muddy. It does not relate to grass or dirt at all. If you click on the Training tab, then hover your pointer over the white square on the right hand side beside each of your horses, it will tell you there if you have a horse that prefers dirt, or will tell you it does not like kick back from the dirt. Also gives recommendations about whether your horse would prefer blinkers, or might benefit from gelding.
I don't believe the stallion or mare has a bigger influence over quality of offspring. I have certainly never noticed any difference.
You cannot predict what price 2yos will sell for at auction. But it is a very good indicator of how good a 2yo is likely to turn out. So if you put a 2yo up for auction and the bidding gets up to very high ($500,000+) then jump in and buy it back. Horses that you buy or sell for less than $250,000 are unlikely to be much more than average hacks.

Thank you for the response!

A follow-up question: Is there any way I can tell if a race track will be firm, fast, muddy etc? (From the weather?)

Not really. When you are in the 'Declarations' section it gives the predicted weather report for the next few meetings, but even if it is forecasting rain or showers it does not necessarily mean the track will be soft or muddy. Often it is still good, or good to soft. Plus it is a real hassle if you are waiting until the last day before nominating horses. A real time waster.
My advice...most of the time the track conditions are good to firm, and the majority of very good horses are suited to these track conditions. Therefore focus on buying or breeding horses that like the better track conditions. Get rid of the others, or if it is a really good horse that likes it wet, then breed it with a horse who likes firm track conditions and you will hopefully get an offspring that will go in good track conditions. Also a really good horse who prefers a better track will still generally run well on the wetter tracks.
I have played SO4 in the UK, USA and Australia flat versions, and the above applies. I have never played the UK jumps, so there may be a case for a different approach with the jumpers.

Thanks for the answer!