Rondo and Dracula X are completely different games that only share the same basic story. It's an unpopular opinion but
I prefer Dracula X (SNES) over SCIV and the PC engine version. I really enjoy the emphasized verticality of the stages and the mobility of the character though it does make the game too easy. Final boss aside, it's also less frustrating than SCIV as it has a more forgiving checkpoint system and you won't die from the screen scrolling up.
If you're interested in Rondo (PC engine) you should get Dracula Chronicles X for PSP (Vita compatible). It's
an ugly but otherwise superior remake of the original, which you can also unlock along with
Symphony of the Night, aka a really boring metroidvania.
The original Castlevania is not that hard so you should be fine given that you beat SCIV, but III is much harder. Still that's my favorite of the series so you should give it a try once you polish you Classivania skills with the easier games and gather enough patience. Just be prepared to repeat some sections over and over
My personal ranking would be:
1.)C III (NES)
2.) Dracula X (SNES)
3.) Dracula Chronicles X (PSP/Vita)
4.) Rondo (PC engine)
5.) C 1 (NES)
6.) SCIV (SNES)