System.TypeLoadException: Could not load type 'UnityEngine.UnityAPICompatibilityVersionAttribute' from assembly 'PlayMaker'

I had to install Unity after a while because App Store asked me to update my game to support iOS x64. I managed to recompile the game using Unity 5. It runs fine but I soon discovered that the main plugin I was using is not working fine anymore because of the new physics. So I tried building it again using Unity 4.x.

The following is the error I get. I have already tried switching player settings to use .NET 2.0 but it still doesn't work. Also tried to switch platform to Android/iOS/PC.

 Internal compiler error. See the console log for more information. output was:
 Unhandled Exception: Mono.CSharp.InternalErrorException: Assets/DFGUI Playmaker Actions/Button Playmaker Actions.cs(10,21): DFGUIButtonGetAutoSize.DFGUIButtonGetAutoSize() ---> System.TypeLoadException: Could not load type 'UnityEngine.UnityAPICompatibilityVersionAttribute' from assembly 'PlayMaker'.
 
   at (wrapper managed-to-native) System.MonoCustomAttrs:GetCustomAttributesInternal (System.Reflection.ICustomAttributeProvider,System.Type,bool)
 
   at System.MonoCustomAttrs.GetCustomAttributesBase (ICustomAttributeProvider obj, System.Type attributeType) [0x00000] in <filename unknown>:0 
 
   at System.MonoCustomAttrs.GetCustomAttributes (ICustomAttributeProvider obj, System.Type attributeType, Boolean inherit) [0x00000] in <filename unknown>:0 
 
   at System.Reflection.Assembly.GetCustomAttributes (System.Type attributeType, Boolean inherit) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeManager.IsThisOrFriendAssembly (System.Reflection.Assembly assembly) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeManager+Closure.Filter (System.Reflection.MemberInfo m, System.Object filter_criteria) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.MemberCache.FindMembers (MemberTypes mt, BindingFlags bf, System.String name, System.Reflection.MemberFilter filter, System.Object criteria) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeManager.MemberLookup_FindMembers (System.Type t, MemberTypes mt, BindingFlags bf, System.String name, System.Boolean& used_cache) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeManager.RealMemberLookup (System.Type invocation_type, System.Type qualifier_type, System.Type queried_type, MemberTypes mt, BindingFlags original_bf, System.String name, IList almost_match) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeManager.MemberLookup (System.Type invocation_type, System.Type qualifier_type, System.Type queried_type, MemberTypes mt, BindingFlags original_bf, System.String name, IList almost_match) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Expression.MemberLookup (Mono.CSharp.CompilerContext ctx, System.Type container_type, System.Type qualifier_type, System.Type queried_type, System.String name, MemberTypes mt, BindingFlags bf, Location loc) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Expression.MemberLookupFinal (Mono.CSharp.ResolveContext ec, System.Type qualifier_type, System.Type queried_type, System.String name, MemberTypes mt, BindingFlags bf, Location loc) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.ConstructorInitializer.DoResolve (Mono.CSharp.ResolveContext ec) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Expression.Resolve (Mono.CSharp.ResolveContext ec, ResolveFlags flags) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Expression.Resolve (Mono.CSharp.ResolveContext ec) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.ExpressionStatement.ResolveStatement (Mono.CSharp.BlockContext ec) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.StatementExpression.Resolve (Mono.CSharp.BlockContext ec) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Block.Resolve (Mono.CSharp.BlockContext ec) [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.ToplevelBlock.Resolve (Mono.CSharp.FlowBranching parent, Mono.CSharp.BlockContext rc, Mono.CSharp.ParametersCompiled ip, IMethodData md) [0x00000] in <filename unknown>:0 
 
   --- End of inner exception stack trace ---
 
   at Mono.CSharp.TypeContainer.EmitConstructors () [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.TypeContainer.EmitType () [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.RootContext.EmitCode () [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Driver.Compile () [0x00000] in <filename unknown>:0 
 
   at Mono.CSharp.Driver.Main (System.String[] args) [0x00000] in <filename unknown>:0