Here's an example of what a Gish Character might have available to them, if they went all out with 4E multiclassing:
For times sake, this is at level 20, the end of your paragon path.
Fighter with multiclass feats and paragon multiclassing as wizard.
Powers: (2 At Will, 5 Encounter, 4 Daily, 5 Utility)
At Will
1 from Fighter List
1 from Wizard List
Encounter
2 from Fighter list
1 from Wizard list from highest level of encounter powers available
1 from Wizard list from 7th level or lower
1 from Wizard list of first level at will powers
Daily
2 from Fighter List
1 from Wizard list from highest level of daily powers available
1 from Wizard list from 19th level or lower
Utility
3 From Fighter List
1 from Wizard list from highest level of utility powers available
1 from Wizard list from 10th level or lower
So as you can see the split of powers is pretty darn even. The Fighter side is 1 utility ability ahead, and a few of the wizard powers have a cap on how high they can be. So the character is more fighter than it is wizard, but its probably about a 60/40 split at most.
If someone wants to argue that this isn't "multi" enough, you can go ahead and try.
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As for the second usual gripe about 4e multiclassing: the opportunity cost. A 20th level character has 12 free feats. A full multiclassed character has 4 multiclassing feats, and 8 free feats. So the versatility gained in multiclassing needs to provide equal or more benefit to a character's 1/3 worst feats at level 20. Thats going to be a judgment call in each situation, but lets go with our fighter/wizard example and list some wizardy powers that the fighter would have that a non-multiclassed fighter would not have access to.
At will from the Wizard List: Let's go with the classic, Magic Missile. Range 20, can be used when given a basic ranged attack, vs reflex.
Encounter power from wizard list: Crushing Titan's Fist, range 20, area burst 2, vs reflex, immobilize targets, squares take 4 squares worth of movement to move through.
Encounter power from wizard list (7th level or lower): Spectral Ram, range 10, vs fortitude, push target 3 squares and knock them prone.
Encounter power from Wizard at will list: Scorching Burst, Range 10, area burst 1, vs reflex.
Daily power from wizard list:Acid wave, close blast 5, vs reflex, ongoing damage
Daily power from wizard list(19th level or lower): Evard's Black Tentacles
Utility power from wizard list: Fly, fly speed of 8, sustain minor (woohoo something to use your minor actions on)
Utility power from wizard list(10th or lower): Arcane gate, create portal that makes two squares within range 20 effectively adjacent to each other, minor action cast, minor action sustain
Is the versatility gained better than just having those 4 feats to use for other stuff? Only time will tell, but its not any where near the "equal level powers are equally useful, so a power swap of the same level doesn't get you anything". I can tell you that a non-multiclassed fighter sure as hell won't be making portals, flying, and immobilizing an area burst 2 from 20 squares away. Gaining the ability to do something that there is no conceivable way you could do otherwise is an increase in power, how much of an increase is still to be determined. But that can only be found in play, not through theorizing on a message board.
All that said, I do think you'd do best to "multiclass" by taking 1-4 of the feats as desired, and taking a paragon path from the other class as opposed to paragon multiclassing. The requirement to have all 4 feats and losing your paragon path doesn't seem to justify the gain from paragon multiclassing. A fighter/bloodmage looks like it could do some cool stuff, although thematically its a bit different from the standard gish. Even if you have to use paragon multiclassing to get the concept you want, it shouldn't be that much worse.