The vocabularly allowed for cadences in AMEB grade 4 theory are the following chords: I, ii, IV, V and vi. You are not allowed to use V7 until grade 6 theory otherwise it would be ideal for the final cadence. You are also allowed to harmonize other parts of the melody with first inversion chords but not the cadences so the chords that you are allowed to use for cadence points will look like this:
You have four cadence points, one at the end the end of each phrase (the phrases are marked by the four slurs) The last cadence must be Perfect or Plagal as both these cadences indicate a final close. In this case it must be a Perfect cadence as chord IV will not fit under the second last note. As chords must be in root position, you place an A in the bass under the second last note followed by a D under the final note, then fill in the two middle voices (doubling the bottom note in each chord)
The other three cadences will be types of imperfect cadence (ie. ending on chord V). I'd suggest using ii-V at the end of the first cadence point (approaching the LN from above), IV-V for the second (make the bass rise a step and the other three fall) and I-V for the third one (which will resemble a "reverse" perfect cadence)
Remember that the best note to double is the root of each chord unless you are doing an interrupted cadence (V-vi) where you would double the middle note in chord vi.