To determine what the property is worth post renovations, you'll want to mostly rely on sales comps. Sub 5 unit is still technically residential and investors won't focus as much on cap rates or gross rent multiples. Find recent fourplex sales and use those as your benchmark.
Going to your other questions:
*How bad are the decks? Did you get a second quote? If you are keeping this as a rental, I'd say splurge and have the decks 100% to avoid any sort of liability in the event something happens in the future.
*To figure out how much to spend on the rehab, you'll need to finalize your comps and determine what returns you want to see while holding the property. Figure out what the market can support so you don't over or under improve.
One thing to consider would be to: take your ARV, subtract your construction budget and holding costs, and price in a risk discount to cover yourself. That should give you a number likely at or above your acquisition basis for the project to make sense. Same idea as when you flip a property.
For example: assuming an ARV of $600k, budget and holding cost of $350k, and a desired profit/discount of 25% to account for the associated risk, I'd come to a desired maximum acquisition basis of $100k. Quick and dirty, still a lot of details to work out.
I'd suggest getting a second and third quote, compile your quotes, make a plan and stick to it. As long as it is structurally sound or repairable to a point that is within budget, I'd suggest keeping it as a fourplex. A little all over the place on guidance but this is really number based and you'll want to compile all of your variables and run some scenarios to see what makes most sense based on your investing criteria. Hope this helps.
