At 95cm your 4yo will most likely not even be tall enough to be in a booster.
Personally I would be getting a 5pt harness seat. The New Maxi Rider is slim has high turn to booster markings and looks easy yo use in booster mode.
I have just moved my 5.5yo to a booster from a new SNS Compaq. That should last to around 120cm in height.
Your boys should be in boosters unless they pass the five step test. Buy HiLiners for themhttp://childrestraints.co.nz/seatbelts.php
The 5 step test will tell if your child is big enough for the adult belt.
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip?
If you answered "no" to any of these questions, your child needs a booster seat to make both the shoulder belt and the lap belt fit right for the best crash protection.Your child will be more comfortable, too!