@Ram Gonzales - I bought a quad earlier this year and in addition to running the numbers as a larger multifamily property (income, expenses, NOI) I would also encourage you to look at the price/unit for triplexs and quads in the area to get a general sense of a comparative market approach. The price/unit in the area will likely be reviewed/analyzed by an appraiser and in fact, could be the main driver of their valuation of the building. It might also give you some insights into why the list price may be what it is... For example, if most quads in the area sold between $600-$650k, then their asking price may not be crazy. However, if most quads sold for ~$500k or so, you know they're likely way off and your offer of $450k might actually be intriguing/interesting.
On another note, if your rent increase estimates are accurate, you'll bring in about $10,200 more on an annual basis (assuming you achieve the $1200 and $1350 rents) so putting in $50k in renovation just passing my gut check of getting about a 20% yield on renovation costs - not great, but not bad either.
Good Luck!