Someone would need to propose a specific law. A law that says 'you may only contract with [insert privileged electronic currency here]" or something similar.

If the enforcement is through confiscation of other currencies it will likely violate 4th amendment protections on personal effects.

If certain contracts are banned (instead of, or in addition to, confiscation) it would violate speech protections. Exchanging money (in any form) is speech, as are your contractual dealings.

An argument as to how restricting the use of cash (or eliminating the use of cash entirely) inherently conflicts with the first or fourth amendments.

> A law that says 'you may only contract with [insert privileged electronic currency here]" or something similar.

The usual method would be more like "All damages, including -- but not limited to -- those for breach of contracts specifying payment in some other currency, shall be awarded and assessed in [privileged currency] as of the date the debt became due and payable."

> If the enforcement is through confiscation of other currencies it will likely violate 4th amendment protections on personal effects.

That's a creative argument, but, no, Fourth Amendment search and seizure protections do not prohibit confiscation of contraband.

> If certain contracts are banned (instead of, or in addition to, confiscation) it would violate speech protections.

Contracts that are contrary to public policy -- such as those whose fulfillment requires an act prohibited by law -- are generally void for that reason; this is a well-settled principal of law and is not contrary to the First Amendment. The idea that the First Amendment means any contract obligation, without restriction, is immune from prohibition is, well, silly in the extreme.