Originally Posted by: leetorts 
It's a license issue. Microsoft has said that games with extensive license agreements most likely will not be backwards compatible. It isn't about them doing the right thing, they can't
Whilst technically true them pointing the finger at licensing being the blocker is a weak argument IMO. It's just a diplomatic way of saying we're too lazy or cheap to make it happen - anything is possible with the right amount of cash.
They managed to release the original horizon on backwards compatibility which notably contains massive amounts of non-original licensed music and nearly an identical manufacturer/brand lineup in terms of cars/parts/etc. The only really notable difference for licensing contraints for horizon vs motorsport is the real life tracks - horizon has none whereas motorsport has many.
So to break it down...
From the music aspect alone horizon would have posed more of a re-licensing issue than forza 4, so i discredit this as being the likely cause of holding back forza 4 backwards compatibility.
From the manufacture/brand aspect i highly doubt any of the partners/alliances would of had a change of heart since the x360 release and are firmly putting their foot down on any re-licensing there use of cars/parts again, in fact in many situations i believe the car manufacturers benefit from being in a game, its free advertising and promotion afterall (there could just be monetary disagreements on how much its worth to either party)
From the track aspect this is the only one that could hold some water. Most notably motorsport 4 had a huge variety of real life japanese tracks which are ALL absent from not only fm5 but fm6 too. This alludes to there being some hiccup or hurdle with t10 being able to put these tracks into the latest games, no idea what but i wouldnt rule out some sort of weird government/country legality between the US and Japan or any number of other weird non-racing related legalities.
But here's the kicker, t10 has now officially stated some of the japanese tracks are set to return in fm7, so one could speculate that whatever hiccup or hurdle there was for fm5/fm6 has now been resolved which then gives less weight to it being track licensing now being the holdup (but doesnt entirely rule it out of course as there are some other tracks/countries involved in fm4).
All in all i think the only real driver (pun!) behind it not yet being released to backwards compatibility is that they haven't deemed it economically beneficial, put simply the cost and effort of doing would exceed the expected return of investment (or in other words they've been too cheap'n'lazy to do it cause it'll cost em more than they'll get back).
However i wouldn't rule out fm4 from being eventually added to the backwards compatibility list... my prediction - right before the launch of fm7 they will give fm4 on backwards compatibility to xbox live gold members to drum up interest and business in the motorsport franchise. This would basically be an identical repeat of the tactic they did for the launch of horizon 3 and the original horizon (assuming they derived that this had a positive outcome for the sales of horizon3 - its hard to quantify this however even for t10).
Anyhoo long stroy short, the potential boost in sales of the new title (fm7) derived from giving the old title (fm4) away to gold members on backwards compatibility could likely outweigh the cost and effort of doing so... and if its going to happen its then or never.. just my hunch ;)