You may not be getting an accurate picture of your sustained write throughput using dd. Since you are using ext4, I'm assuming you are using Linux :-) Linux does a good job caching to RAM before writing to disk, so when you write only 1GB using dd, you are mostly exercising your RAM (assuming you have a normal amount of RAM).
If you run a test writing a file much larger than your RAM, you should see something closer to your true sustained write speed.
For example, I get the following results running your test, which make this caching very obvious:
[ Desktop]$ dd if=/dev/zero of=/home/amadsen/test.bin bs=10K count=100000 100000+0 records in 100000+0 records out 1024000000 bytes (1.0 GB) copied, 2.62228 s, 391 MB/s
If I increase the amount by a factor of 10, the system is forced to actually write to disk finally and the throughput falls:
[Desktop]$ dd if=/dev/zero of=/home/amadsen/test.bin bs=100K count=100000 100000+0 records in 100000+0 records out 10240000000 bytes (10 GB) copied, 77.4247 s, 132 MB/s