You need a T-ring (specific to your Canon camera body), and a T-adapter (mostly generic) that mates to back of the 4SE. The latter is threaded on both ends, one end mates to the visual back of the OTA, the other threads into the inboard end of the T-ring. The T-ring has a bayonet adapter that mates to the camera.
With that in mind, the 4SE is a very lightweight alt-az, single-fork mounted scope. It is asking a lot to attach a DSLR to the back of this and expect to get any sort of high performance out of the combination.
A Barlow is a sub-aperture corrector that acts like a tele-extender. It increases the effective focal length of the telescope, which in turn increases magnification (for visual use) of the telescope/eyepiece combination, or increases image scale in the case of imaging. Somtimes a Barlow is used in combination with a web-cam/video imager for planetary imaging, to increase the apparent image size. The usual case with an SCT for most other imaging applications is the opposite - the inherent focal length is too long, so a focal reducer is used to increase the apparent field of view.
You might use the T3i's video mode for planetary imaging, however there are better, less expensive and clumsy solutions based on web-cam/dedicated video imagers.
It is unlikely you will do much DSO imaging with the 4SE and any camera combination - due in part to the alt-az mount, and in part to a number of other factors including the relatively unstable tripod, etc.
The 4SE is basically intended for visual use, not for imaging...
CGEM 800 HD, NexGuide, Orion XT8 Limited Edition, Oberwerk BT-100, Canon 20D/20Da/T3i/60D/5D Mk III, various eyepieces, adapters, geegaws, widgets, and tiddlybits