I do not know which part of the world you live in - Britax is an Australian company which has been a world leader in child safety restraints for many years and has a full range of products to accommodate new-borns right through to 12 year olds. They have international distributors who can be accessed through the following link:
http://www.britax.com.au/websites/Br...e-CONTACT%20US
The most appropriate product for your 4 year old would be their Nova, Mobility or Horizon booster seat which are designed to be used with a center lap belt - see the following link for more specific information:
http://www.britax.com.au/websites/Britax/HomePage.nsf/LUPages/SnS-BOOSTERS - 14KG TO 26KG
My four adult children and two grandchildren [so far] have used Britax products in cars ranging from a Ford Capri to a Corniche Drophead and my experience with them has exceeded every expectation
