The SE4 is a four inch Mak. It does a great job on the moon and planets. It is pretty gutless on DSO's, especially under light pollution (your signature doesn't state where you live).
I have tons of experience with the Celestron SE and Evolution mounts. And no experience with the SLT's.
It is my understanding that the SLT was the first one introduced. It was followed by the SE, the the EVO. Each mount was improvement on the earlier one. I would be skeptical of purchasing a SLT.
The best starter scope is probably the EVO6 --- an awesome mount with a great general purpose OTA. Alas, it's a budget buster.
At a bare minimum, consider a SE5. The SE mount is a step up and the 5 inch SCT is a better overall OTA than the 4 inch Mak . The 5 uses a standard visual back and diagonal and the 5 enough light grasp for DSO's.
I am just not a fan of the Celestron SE4 optical tube. It is gutless for DSO's. So, you are limited to planets and the moon. Instead of visual back threads, the 4 has a Micky Mouse built in flip mirror so you can use it straight thru or at 90 degrees. I think the tube looks clunky. And good luck cleaning the mirror when it gets dirty/dusty.
I don't mean to bad mouth the SLT 4. Heck, one can do a lot more observing with one than without one. But, just keep in mind that it is an economical starter scope with engineering and cost tradeoffs.