Oh yes, indeed I did play Final Fantasy IV back in the day. It was one of the first two SNES games I got and a major memory from my early years.
You're considering buying the VC port of Final Fantasy IV for the SNES and want to know if you'd enjoy it, via gameplay comparison to FF3 and FF6. Well, let me see if I can help you.
Well, the ultimate difference between FF3 and FF4 is the Jobs system: There isn't one. Instead of a team of four customizable Onion Knights, all the characters in FFIV have set classes, which cover the standard spectrum of FF jobs (including Dragoon, Summoner, White Mage, Black Mage, etc). They are cycled in and out of your party according to storyline, rather than your whims, and have their own dialogue and characterization (which drives the story).
FF4 was highly innovative for it's time. The battle system is no longer the turn-based NES model. It is, in fact, the debut of the Active-Time Battle system, pretty similar to 6. Your party size grows and shrinks throughout the game and eventually maxes out at five(!).
In order to keep things interesting, FF4 really pioneered (and ran with) the new concept of the Gimmicky Boss Battle. Nearly every boss battle (and a few random ones) has some special gimmick (or trap), frequently requiring some new tactic (such as taking them out in order, using a particular technique at the right time, or in one notable example, not attacking at all). Battles are usually fast-paced and crisp, unencumbered by long, drawn-out attack animations.
As for dungeon layouts, this was back in the golden era of maze-like dungeons (no ultra-linear FF9 guided tours here) with ample rewards for exploration. Exploring everywhere, even in towns, often will yield secret passages full of extra goodies, and even one or two memorable easter eggs.
Aside from the main progression, there are a handful of optional side-quests which an exploratory player can locate, which provide extra difficulty but often worthwhile rewards.
As to challenge, the US version was deliberately toned-down. Hidden tunnels in walls (invisible in Japanese versions) have also often been graphically highlighted and obvious, to save you some tedium. The encounter rate was lowered, and as long as you don't run away too frequently, you will usually have enough levels to keep moving forward. There will be some periods, especially early on but not uncommon afterward, where you'll find yourself stopping to do some fund-raising with the local monsters, to keep up your equipment. Speaking of equipment, save often, or fall victim to Wutai Theft whenever someone leaves the party unexpectedly.
It really is an early 16-bit SNES RPG, but if I may say, it is the best of the bunch, less tedious than any other. This really was my favorite game for several years (til FF6 came out), and if you want a classic, you really can't do better than FF4.
However, if I may say though: FF4 really is an older game, and a remastered update, for the Nintendo DS, was released not long ago. It featured 3-D characters and environments, extra bonus dungeons, a party you can eventually customize, considerably punched-up dialogue, and even restored some elements of the game that were removed entirely from the SNES version in the US.
If you're going to go for the full FF4 experience, less dated, I would suggest you consider that one. But, if you just want a classic, old-timey RPG experience for a few bucks, go for the VC one. Your choice. Just pointing it out.
Let me know if there are any other points you'd like clarification on.
"Any plan vere hyu lose hyu hat iss a bad plan."
~Ancient Jagermonster proverb