Insert performance increases under load: Why?
One possible reason is that four concurrent processes generate a more favourable pattern of log flushes - typically meaning that each log flush writes more data than is the case with a single executing process.

To determine if transaction log throughput/flush size is a factor, monitor:


sys.dm_os_wait_stats for WRITELOG and LOGBUFFER waits
sys.dm_io_pending_io_requests for IO performance
Performance Monitor counters (or sys.dm_os_performance_counters) for:


Log Bytes Flushed/sec
Log Flushes/sec
Log Flush Wait Time



Look for internal limits being reached. In SQL Server 2008 R2, there can be a maximum of 32 outstanding (asynchronous) log flush I/Os per database on 64-bit versions (only 8 on 32-bit). There is also a total size limit on the outstanding IOs of 3840KB.

More information and further reading:


Transaction Log Monitoring
Trimming Transaction Log Fat
Diagnosing Transaction Log Performance Issues and Limits of the Log Manager
Optimizing Transaction Log Throughout