We use a software system called Builder Trend so that each and every investor can track the progress of their property. 

Looking at public records for these 4 plexes (there are probably 30 or so in the area) the sale prices are all over the place regardless of the year sold (there is no clear trend of values going up or down) so I suspect they are generally selling based on income, condition and motivation of the seller.

You can easily hedge for it by buying intelligently, taking advantage of micro-trends, remodeling the property etc.Depending on how quickly your coffers refill, I would tell you to buy in a market that will provide good return on your funds, and to keep a close eye on your CA market until this next down cycle has played out (maybe 3-4 years?).

I'm the inventor of Bollinger Trend PRO forex indicator for mt4 trading platform.

If I invest in rehabbing these older houses, should I expect the house to last another 100 years with the proper maintenance, or should I consider a house's life expectancy to always be on a downward trend because maintaining it is no longer cost-effective?

I am a Mortgage banker ( inactive) but I try to keep up on trends.

A good agent is what you'll need to follow the trends and especially to get your property sold.

Maybe start investing "passively" through S&P500 funds that will mimic the stock market trends so you can grow your savings (this would be more of a long term play and just make sure the fund has low expense ratio.)   

The trend is to open retail and offices where there is lots of traffic like malls. 

You need to look at violent versus non-violent crimes and if trending upward.You can have the best property in a marginal area and have constant problems.It's better if you reverse it and buy in a B area close to you with C- tenants.