Reconsider that cell phone for your 4 year old. 

A September 2009 article in Surgical Neurology looked at 11 long-term studies and found that "using a cell phone for >10 years approximately doubles the risk of being diagnosed with a brain tumor on the same side of the head."  That's scary, especially for children, who are starting much earlier than we did.

Cell phones emit microwave radiation.  There are studies from 7 countries now that show cell phone emissions can break DNA strands or alter it structurally.  I don't think it's a coincidence that the Central Brain Tumor Registry has reported a 36% increase in brain tumors in less than a decade.

The effects add up over time, so starting young increases the danger.  The risk is already greater for children because of their thinner skulls, higher water content in the brain (remember that's how a microwave heats, it acts on water), and smaller brains.  According to studies I've seen, a cell phone call of only two minutes can alter the electrical activity of a child's brain for up to an hour!

The most dangerous area is the 4 to 5" radius around the antenna, so holding the phone even an inch further from your head will make a difference.  Power intensity increases as the signal gets poorer, so exposure is greater if you're in a metal building, a car, or a low coverage area.  Power is also higher while the call is first being connected.

Conventional wired headsets may reduce exposure.  Bluetooth headsets reduce power intensity, but they operate in the same frequency range as microwave ovens;  because they're relatively new, we really don't know if they're safer.. or more dangerous. 

There are headsets hitting the market now that are like the old airline headsets, where the sound travels through the air in a tube.  These airtube headsets may be the safest option.  They're easy to find on the internet.

What else can you do?  Hold the kids off as long as possible... my 17 year old quadruplets didn't get cell phones until their 17th birthday.   When kids do get a phone, educate them about the risks and limit their use. 

Use a wired land line when available (cordless phones are also a risk); hold the phone away from you when first connecting and avoid long conversations on a poor signal. Use speakerphone when possible, alternate ears, and don't talk when you can text. 

You can check the relative safety of your phone emissions at www.ewg.org/cellphoneradiation/Get-a-Safer-Phone.