Well, I’m no lawyer but, as far as I know, you could “clone” 4e; but, in order to do that, you actually need to create your own system that looks like 4e. It’s a great amount of work just for a clone, especially when the original wasn’t received with golden crowns by its fanbase, so to speak.
All people who would work in such a game would pile huge amounts of changes just to reflect their personal preferences – I speak from personal experience. And every 4e fan has his/her own ideas about what to change.
Plus, even if it is completely legal to do what I said, WotC would sue him/her, and this would cost much more money just to defend himself/herself, even if the defendant is right!
It’s too much risk and work for something which wouldn’t be that worthy – this seems to be the designers’ opinion.
Its hard to see why WotC would sue. Its hard to say that they would lose if someone actually took it to court either, it depends on the specifics and there are a number of complex points.
However, I don't really think WotC WOULD sue you. Not unless you really seriously wholesale copied from 4e in a big way, so that your game really wasn't at all distinct.
For instance I wrote my own game. It is quite '4e-like'. There are similarities at many levels, and I have called it a '4e hack' informally. Its still a distinct game, with distinct features of play and its own design goals. While it does things like grant characters 'powers', which conform to standardized block formats and are granted in ways roughly similar to what happens in 4e none of them duplicate 4e powers in name, exact format, etc. Likewise there is a monster stat block format, analogous to ones used in 4e, though the stats are somewhat different and the layout is different. The point is, its a game that 4e fans would probably say is "close to 4e, spiritually and mechanically", but it is also a distinct game. I don't really see any reason why WotC would care about it. Maybe if it proved to be a great commercial success they might care more, but I seriously doubt it, 5e seems to be all they had hoped for. It behooves them little to make a rep as jack booted IP enforcers in an industry where everyone is cribbing off each other a whole lot already.
Truthfully, I just don't see a huge business case for a 4e 'clone'. Even if it was modestly popular its a very material-intensive system that requires a LOT of development, and is unlikely to be more than modestly popular (like all 3PP games, maybe PF excepted, have been in the last 20 years). If I was going to write a game to make money, I'd aim at a much lighter-weight system, one that could be produced as a single book of modest size and aimed at covering some niche subject matter that happened to be current. Print a couple 1000 copies, sell as many PDFs as you can, do it all through kickstarter, and be done with it. You won't make loads of money, but its a fairly conservative and viable strategy if game design is your thing.
Paizo managed to pull off cloning 3.5 under very specific circumstances. That is VERY unlikely to be a generally successful strategy. I doubt we will see such a thing again any time soon.