Hi @Justin McElroy. Phew!!! It's like the question, what came first, the chicken or the egg, right? First, let me congratulate you about getting your hands dirty, Not everybody is courageous enough to take action.
I understand your reluctance to put real estate under contract if you don't know your numbers. 4 units is still considered as residential and you should be good to go with residential numbers (although I would be very precise in this case). Could you find an experienced investor in your area who would be ok to help you out, even if you have to split the profit? Or could you at least make a list of the needed repairs and submit it to someone with experience?
I would also recommend that you spend time at Lowe's, Home Depot and such and start to look into all the material's prices. Don't forget the small stuff. It will allow you to get an idea of what things cost. It will not be totally right, but close enough.
You could also get to know a GC and offer to pay him for an estimate. Be careful though, all GC's are not, how to put it... straight in their boots. Most of them are good people, but some could take advantage of your lack of knowledge. Once again, get in touch with people who have experience and can recommend someone.
Over time, you'll gain experience and you will be able to estimate the repair costs without help. There is also a good book about it in Bigger Pockets. Visit their store and you'll find it. (the title escapes me now, but it is obvious when you read it).
Good luck and success!