This is the gearing on the SL5 with 105 drive train:

11-34 gearing 11-13-15-17-19-21-23-25-27-30-34T

And on the SL4 with Tiagra drive train:

​​​​​​12-13-14-15-17-19-21-23-25-28T

The OP is talking about three things that combined make the $500 price difference, not just one.

If one isn't heavy, (or worse, heavy and old like me), and also doesn't do any serious climbing of a couple of miles or more of category 3 or higher especially with some double-digit grade, (like 11-18 percent) than a 28T certainly would be fine.

But also the SL4 10 speed, 28T cassette has as its hardest gear a 12T on the Tiagra as opposed to an 11T on the 105.

Finally, IMO the worst thing gearing wise is to run out gears on a climb or long ride. One IMO should always be liberal in choosing their top of the cassette gearing. I do a lot of climbing that includes sections on cat 3 and 2 that include sections that run 11-18 percent.