I don't know. The cheapest non contract phone I've seen starts at $50 per. So, that's $200 for 4. Assuming that you can get one that fits your needs (where I live only certain carriers have actual reliable coverage at my home.)
We are with Verizon, and while we only have one smart phone and one "plain" one (text and talk service only) our bill is $125. Because they charge a flat fee for "sharing" and we have unlimited talk/text to share. But, I could add 2 smart phones for $40 each... and that would make my bill about $205/month for 3 smart phones and one plain. But I don't carry "extras" either. I have never had a problem with broken phones or lost phones or other things that would make me feel like paying $50/month in insurance on them is worth it.
I have the smallest data package available (1G) and I NEVER come even close to using it. Maybe 10% of it gets used per month.
oh.. and I can use my phone as a mobile hotspot, so I don't have to add my ipad as another device to the plan. I just turn on the hotspot if I am someplace without wifi and I want to use it online.