"Real multiclassing" is either Dabbling or a Mashup, in a broad stroke. Dabbling we have, and Mashups are supposed to be actual classes anyway. There might be a third option between the poles, but I'm not seeing a distinction.
Okey-dokey, here's your distinction.
A 3.X PC multiclasses into Cleric. By doing so, he gains access to all of the skills the Cleric has as class feats. He can cast any Clerical 0 or 1st level spell he selects. He has access to all of the weapons and armor proficiencies of the Cleric. He has access to the Cleric's HD. He can Turn Undead. All this as early as 2nd level. He will also qualify for all PrCls requiring the PC being a Cleric. By 4th level, this PC will have a suite of spells to use going up to 2nd level.
Consider the first multiclassing feat in the 4Ed PHB, Initiate of the Faith, which grants access to one specific Cleric skill (Religion) and the ability to use healing word once per day, and can use a holy symbol as an implement when using a cleric power or cleric paragon power. In addition, you gain access to any feat for which "Cleric" is a prereq and for the Power-Swap feats. However, you don't gain the Cleric's HP or weapons, his # of Healing Surges. You can't cast any spells. And most important, there are no feats in the 4Ed PHB that require "Cleric" as a prereq. The feats that are "clerical" in nature all require the "Channel Divinity" class feature that this feat does not grant. Barring some kind of Feat in an as-yet-unpublished supplement (IOW, cash up some more $$$), he will
never be able to Turn Undead or any of the other uses for CD. The Power Swap feats aren't available until 4th level or higher. RAW, he cannot qualify for a Cleric paragon path (IotF qualifies you for "feats"- it doesn't say you are considered a cleric), meaning the only way you can get access to cleric or paragon powers to get any use out of your holy symbol is by burning yet another feat for a Power-Swap...which you won't get until at least 4th level.
The second (4th Ed) PC is clearly getting less bang for the buck, and is arguably
gimped.
FURTHERMORE
4Ed style multiclassing is unidirectional, and only supports one kind of PC concept- the Dabbler.
3.X allowed for a variety of PC concepts. The Dabbler is supported, but so is the Renaissance Man, the Jack of All Trades (and Master of None), and the person who has experienced a profound change in his life.
The last one is the most telling- it is a powerful archetype in myth and fantasy lit, as well as reality. For example, it was not uncommon for warriors in feudal Japan to retire from the way of bushido and enter a monastery. In the West, entry into a monastery or cloister was another relatively common way for someone to change their lives, even after a lifetime of achievement.
A thug who "sees the light," like a killer who becomes a moral pillar and reforms himself is another relatively common trope.
And it is utterly unsupported by 4Ed's multiclassing rules. You choose your PC's first class, and he is primarily that
forevermore.
