To my knowledge, Silicic acid actually refers to a group of acids formed from compounds that are composed of silicon oxides bonded to hydroxyl groups. The acids that you've stated, H4SiO4 and H2SiO3 are both known as 'silicic acid" (the name of the group) though their actual names are orthosilicic acid and metasilicic acid respectively.
H4SiO4 however is much more common, so maybe that's why your source is equivalating silicic scid to orthosilicic acid.
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