SQL Server 2008R2 minimum RAID/Disk config
For a 4 bay configuration with a high insert/update workload I would go with one of these:

1) Spindle only


One RAID1 for the log. Back this up with a a few hundred MB of write cache
One RAID1 for the data files and tempdb


Rationale: you will mostly be writing to the log. Latency of log is king for OLTP and it doesn't get much better than a RAID1 with a write cache. You can use SATA - which should give you at least 80MB/sec sequential writes at latency less than 1ms.

2) SSD plus spindle


One RAID1 for log with same configuration as the above.
One RAID1 for data files and tempdb


Rationale: This config will favour higher tempdb load and would be good if you are skimping on DRAM (as this will cause checkpoint activity to go up and drive read I/O)

3) Pure SSD


Put them in a RAID5 if you are not worried about wearing them out. 
If you are using consumer grade SSD - put them in a RAID10.


Rationale: RAID5 with 4 x SSD is fast enough for nearly all purposes. 

Everything goes on same LUN

4) 8 bay, spindle only config


Put 2 disks in RAID1 for the log file.
Rest goes into a RAID10 or RAID5 (RAID10 if you are worried about tempdb)


Overall: always prefer a 2 spindle RAID1 with a write cache for logs. It's plenty for 99% of all cases (80MB/sec for SATA and up to 120MB/sec for SAS).