I don't think /inputdevice=mouseover will work for this, at least not currently. However, here's a way to make do without it:
<expressions>
/ ismouseovera = (mouse.x > 0.05*display.width && mouse.x < 0.35*display.width && mouse.y > 0.35*display.height && mouse.y < 0.65*display.height)
/ ismouseoverb = (mouse.x > 0.65*display.width && mouse.x < 0.95*display.width && mouse.y > 0.35*display.height && mouse.y < 0.65*display.height)
</expressions>
<block myblock>
/ trials = [1-4=sequence(centermousetrial,mouseovertrial)]
</block>
<trial centermousetrial>
/ stimulusframes = [1=clickhere]
/ inputdevice = mouse
/ validresponse = (clickhere)
</trial>
<text clickhere>
/ items = ("Click here!")
</text>
<trial mouseovertrial>
/ posttrialpause = 500
/ stimulusframes = [1=a,b]
/ inputdevice = mouse
/ validresponse = (mousemove)
/ isvalidresponse = [expressions.ismouseovera || expressions.ismouseoverb]
</trial>
<text a>
/ items = ("A1", "A2", "A3", "A4")
/ txbgcolor = (red)
/ size = (30%,30%)
/ position = (20%,50%)
</text>
<text b>
/ items = ("B1", "B2", "B3", "B4")
/ txbgcolor = (green)
/ size = (30%,30%)
/ position = (80%,50%)
</text>