Mark,
Direct "equivalent" Intel controllers to the SROMBSAS18E are the SRCSAS18E, SRCSAS144E and SRCSASPH16I. The SROMBSAS18E is a module where the others are discrete controllers that plug into PCIe slots. You can find more information on each if you go to http://www.intel.com/p/en_US/support and search for the controller. The Technical Product Specifications are good resources for information.
Looking at the compatibility list for the SR4850HW4, not many SAS controllers are listed. Even less PCIe ones. The ones that are listed are LSI Logic* 1068 SAS ROC controllers, like the SROMBSAS18E.
I think you're going to have a hard time finding the 1068 controllers. Most have ceased being manufactured.
The SAS backplane in the SR4850HW4 uses a SFF8087 connector. This is pretty common among RAID controllers. SFF8087 to SFF8087 cables should be available almost anywhere.
As noted, the SROMBSAS18E or SRCSAS18E uses a LSI Logic* 1068 SAS ROC. From this thread subject, the SRCSASRB uses the LSI Logic* 1078 SAS ROC. A 1078 controller should work, but I don't know if it'll fit or exact compatibility because we haven't validated it with the SR4850HW4.
If you choose to try, I can't express the importance of a backup enough in case something goes wrong.
The generic process to follow would be to install the new controller and reset it's configuration without any connection to the backplane/disks. This will eliminate the "import foreign configuration" encountered if you have different configurations on the controller and disks (array). Obviously you want the configuration on the existing array. Once the configuration is reset, power off and connect the drives and power on. If all goes well, the DDF configuration from the drives should be imported automatically by the RAID controller and you'll be up and running.
If something doesn't look normal, stop. You don't want to create more issues. This is where a backup comes in handy. You could just recreate the array with the new controller and restore your data.
Good Luck,
John