diff --git a/CurrentVersion.props b/CurrentVersion.props
index 0a9d3e1d..3739d1b8 100644
--- a/CurrentVersion.props
+++ b/CurrentVersion.props
@@ -1,11 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <MajorVersion>1</MajorVersion>
-    <MinorVersion>1</MinorVersion>
-    <MicroVersion>4</MicroVersion>
-    <ReleaseLevel>alpha</ReleaseLevel>
-    <ReleaseSerial>0</ReleaseSerial>
+    <MajorVersion>4</MajorVersion>
+    <MinorVersion>0</MinorVersion>
+    <MicroVersion>0</MicroVersion>
+    <ReleaseLevel>preview</ReleaseLevel>
+    <ReleaseSerial>1</ReleaseSerial>
 
     <AssemblyRevision>0</AssemblyRevision>
     
diff --git a/README.md b/README.md
index c7de23ea..58da9437 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ Ruby 1.9 grammar it shipped with in 2011.
 ```console
 $ ./irb.sh
 irb(main):001> RUBY_DESCRIPTION
-=> "IronRuby 1.2.0-dev (4.0.0) on .NET 8.0.31 [x86_64-linux]"
+=> "IronRuby 4.0.0-preview1 (4.0.0) on .NET 8.0.31 [x86_64-linux]"
 irb(main):002> def fib(n) = n < 2 ? n : fib(n - 1) + fib(n - 2)
 => :fib
 irb(main):003> (1..10).map { fib(_1) }
@@ -58,6 +58,17 @@ def connect(host:, port: 5432, **opts) = Client.new(host, port, **opts)
 users&.filter_map { it.name if it.active? }
 ```
 
+## Version numbers
+
+IronRuby's version follows the Ruby version it implements: **IronRuby 4.0 is Ruby 4.0**,
+and it becomes 4.1 when it moves to Ruby 4.1. The last number counts IronRuby's own
+releases for that Ruby version (4.0.0, 4.0.1, ...) and is not CRuby's patch level; the
+release notes say which CRuby release the specs were checked against. The last release of
+the original project was 1.1.3, for Ruby 1.9.2, in 2011.
+
+`IRONRUBY_VERSION` is the full version (`4.0.0-preview1`), `RUBY_ENGINE_VERSION` the same
+without the pre-release suffix, and `RUBY_VERSION` the Ruby version (`4.0.0`).
+
 ## What changed
 
 | | before | after |
@@ -440,7 +451,7 @@ Windows machine, keeping the same relative layout (`ir.cmd` finds the binaries u
 ```
 C:\ir> set IR_CONFIG=Release
 C:\ir> ir.cmd -e "puts RUBY_DESCRIPTION"
-IronRuby 1.2.0-dev (4.0.0) on .NET 8.0.31 [x64-mswin64]
+IronRuby 4.0.0-preview1 (4.0.0) on .NET 8.0.31 [x64-mswin64]
 C:\ir> irb.cmd
 C:\ir> set RUBY_EXE=C:\ir\ir.cmd
 C:\ir> ir.cmd -Imspec/lib mspec/bin/mspec-run spec/language
diff --git a/Src/Ruby/CurrentVersion.cs b/Src/Ruby/CurrentVersion.cs
index 1a1f730d..8f593b1f 100644
--- a/Src/Ruby/CurrentVersion.cs
+++ b/Src/Ruby/CurrentVersion.cs
@@ -1,25 +1,29 @@
 // Hand-maintained. The legacy build generated this from Build/Templates plus
 // CurrentVersion.props; the modern SDK build has no such step.
+//
+// Major.Minor is the Ruby version IronRuby implements: IronRuby 4.0 is Ruby 4.0, and
+// moves to 4.1 when RUBY_VERSION does. Micro counts IronRuby's own releases for that
+// Ruby version; it is not CRuby's patch level.
 using System;
 
 namespace IronRuby {
     public static class CurrentVersion {
-        public const int Major = 1;
-        public const int Minor = 2;
+        public const int Major = 4;
+        public const int Minor = 0;
         public const int Micro = 0;
-        public const string ReleaseLevel = "dev";
-        public const int ReleaseSerial = 0;
+        public const string ReleaseLevel = "preview";
+        public const int ReleaseSerial = 1;
 
-        public const string ShortReleaseLevel = "dev";
+        public const string ShortReleaseLevel = "preview";
 
-        public const string Series = "1.2";
-        public const string DisplayVersion = "1.2.0-dev";
-        public const string DisplayName = "IronRuby 1.2.0-dev";
+        public const string Series = "4.0";
+        public const string DisplayVersion = "4.0.0-preview1";
+        public const string DisplayName = "IronRuby 4.0.0-preview1";
 
-        public const string AssemblyVersion = "1.2.0.0";
+        public const string AssemblyVersion = "4.0.0.0";
 
-        public const string AssemblyFileVersion = "1.2.0.0";
-        public const string AssemblyInformationalVersion = "IronRuby 1.2.0 dev 0";
+        public const string AssemblyFileVersion = "4.0.0.0";
+        public const string AssemblyInformationalVersion = "IronRuby 4.0.0 preview 1";
 
         public static readonly Version Version = new Version(Major, Minor, Micro);
     }
diff --git a/Src/StdLib/ironruby/ruby4.rb b/Src/StdLib/ironruby/ruby4.rb
index c183f7cb..40ab1c47 100644
--- a/Src/StdLib/ironruby/ruby4.rb
+++ b/Src/StdLib/ironruby/ruby4.rb
@@ -1627,7 +1627,10 @@ def __pm_find_index__(arr, count)
   end
 end
 
-RUBY_ENGINE_VERSION = RUBY_VERSION unless defined?(RUBY_ENGINE_VERSION)
+# IronRuby's own version, without the pre-release suffix: 4.0.0 for 4.0.0-preview1.
+# IronRuby's major.minor is the Ruby version it implements (CurrentVersion.cs), so this
+# agrees with RUBY_VERSION up to IronRuby's own release counter, as it does in CRuby.
+RUBY_ENGINE_VERSION = IRONRUBY_VERSION[/\A\d+\.\d+\.\d+/].freeze unless defined?(RUBY_ENGINE_VERSION)
 RUBY_COPYRIGHT = "ironruby - Apache License, Version 2.0".freeze unless defined?(RUBY_COPYRIGHT)
 RUBY_DESCRIPTION = "ironruby #{RUBY_VERSION} (.NET)".freeze unless defined?(RUBY_DESCRIPTION)
 
diff --git a/Util/package-release.sh b/Util/package-release.sh
index bf32cb70..d6401039 100755
--- a/Util/package-release.sh
+++ b/Util/package-release.sh
@@ -3,7 +3,7 @@
 #
 #   Util/package-release.sh linux-x64            # -> dist/ironruby-<version>-linux-x64.tar.gz
 #   Util/package-release.sh win-x64 out          # -> out/ironruby-<version>-win-x64.zip
-#   IR_VERSION=1.2.0-preview1 Util/package-release.sh osx-arm64
+#   IR_VERSION=4.0.0-preview1 Util/package-release.sh osx-arm64
 #
 # The archive keeps the source tree's layout - the launchers at the top, the build in
 # Src/Console/bin/Release/<tfm>, the standard library in Src/StdLib - so ir.sh/ir.cmd,