anyone could guess you wrote it

[Newton]

In a classroom, one student (A) to another (B):

A: I'm surprised! How did you know I wrote it on the board?

(The writing is not on the board now. Maybe someone erased it, but B had already seen it.)

B1: It's pretty simple. Looking at the handwriting, anyone could guess you wrote it. (Guessing in the present)

B2: It's pretty simple. Looking at the handwriting, anyone would guess you wrote it. (Guessing in the present)

B3: It's pretty simple. Looking at the handwriting, anyone could've guessed you wrote it. (Guessing in the past)

B4: It's pretty simple. Looking at the handwriting, anyone would've guessed you wrote it. (Guessing in the past)

Are B1, B2, B3 and B4 idiomatic/correct in the given context?

[Alan]

Yes, all possible given the appropriate context/background knowledge.