I started playing guitar 10 years ago at age 62, I now play rhythm in a small country band. I would like to know all the things I can do on the first four frets to embellish my chords, ie., inserting individual notes, playing bass runs and scales, and possibly playing leads. I have seen other rhythm players doing this and believe this would be a good first step beyond where I am now. Can you help me with that?
As Dr Mayhem's comment states, your question is very broad, so I will give a broad answer.
Learn all the scales in all the keys in all positions.
Learn all the chords and arpeggios in all the keys in all positions.
Learn all the songs and the associated techniques that do the things you want to do.
The first two may take 500 hours of practice, the last could take many lifetimes to achieve so I'll reduce it to fit more with your goals based on my limited experience of country music:
learn the common scales in the common keys in first position. Common scales in country are the Major, pentatonic major, mixolydian, major blues, minor, dorian. Common keys are the usual guitar centric keys C, G, E, D and their relative minors.
Learn the common chords and arpeggios in the common keys in first position. common chords in country are Major, Minor, Dom7, 6, dim, aug, Maj7, min7.
And most importantly, learn the songs that do what you want to be able to do. This is where you get to see how the scales and chords are used and the techniques used to apply them. Some important techniques are string bending (very common in country!), hammer ons, pull offs, slides, hybrid picking (and slide playing..) You will also see the common progressions and common substitutions and variations on those. And also the cliche moves you'll hear again and again eg ascending chromatic bass runs up the root of the next chord, hammering or bending from the minor third to the major third, root fifth alternating basslines and various other country idioms.
This clip gives an thorough overview of techniques you'll likely come across again and again.
As I said I'm no country guru so the common scales and chords are not exhaustive.