• Integrated support for "go to definitions" in Merlin.
• Standard library: new modules In_channel and Out_channel, many new functions in Seq module, UTF decoding and validation support for strings and bytes.
• Runtime optimisation: GC prefetching. Benchmarks show a speedup of around 20% in GC-heavy programs.
• Improved error messages in particular for module-level error.
• Deprecated functions and modules in preparation for OCaml 5. In particular, the Stream and Genlex modules are now deprecated.
• Type variables can be explicitly introduced in value and variant constructor declarations. For instance,

    val fold: ('acc -> 'elt -> 'acc) -> 'acc -> 'elt list -> 'acc
    type showable = Show: 'a * ('a -> string) -> showable

can now be written as

    val fold: 'acc 'elt. ('acc -> 'elt -> 'acc) -> 'acc -> 'elt list -> 'acc
    type showable = Show: 'a. 'a * ('a -> string) -> showable

• Tail-call with up to 64 arguments are now guaranteed to be optimized for all architectures.
• Experimental tail modulo cons (TMC) transformation