Pop in G (4 chords) — alternating bass fingerstyle
Of the four chords, only G and Em offer a genuine alternating-bass option: both their low E and A strings are playable, giving two distinct bass notes to alternate. D and C each have only one bass string (the low E string is cut on both): the thumb necessarily stays on the same string there, despite the technique's name.
Tips
On G and Em, consciously work the alternation between the two available bass strings.
On D and C, don't look for an alternation that doesn't exist: focus on the thumb's regularity on its single bass string.
Compare with the catalogue's blues grid (D7, G7, A7): the same split between chords with real alternation and chords with a single bass string shows up there too.
Chord progression — G
Bar 1
G
I
Bar 2
D
V
Bar 3
Em
vi
Bar 4
C
IV