Although there are answers in the comments, I thought I'd provide an official one.
You're correct that the music has not yet modulated to D, as evidenced by (among other things) the C♮ that appears in the bass at the end of m. 4.
Measure 4 is clearly a V chord (D), and that C♯ clearly suggests a tonicization of that V chord. When we stack that chord in m. 3 in thirds, we get
C♯–E–G, which is a vii° triad (the ° means diminished) in the key of D. With the E in the bass it's in first inversion, so this chord is best called a vii°6/V (read "seven-diminished six of five"), which is a very common occurrence in step-descent basses like this one.
And as some of the comments above discuss, if one wishes to include the A in the chord, we're left with
A–C♯–E–G, which is a V7 chord in D, but now in second inversion. If you include the A, the chord is then best labeled V43/V.