Source code/ ViewModel: http://stackoverflow.com/questions/20400257/remove-the-item-which-is-selected-in-other-combobox-from-source

gistfile1.cs

using System.Collections.ObjectModel;
using System.ComponentModel;

namespace WpfApplication1.ViewModel
{
    public class MainWindowViewModel : INotifyPropertyChanged
    {
        public MainWindowViewModel()
        {
            InitSourceCollection();
        }

        public ObservableCollection<People> SourceCollection1 { get; set; }

        public ObservableCollection<People> SourceCollection2 { get; set; }

        public ObservableCollection<People> SourceCollection3 { get; set; }

        public ObservableCollection<People> SourceCollection4 { get; set; }

        private People _selected1;
        public People Selected1
        {
            get
            {
                return _selected1;
            }

            set
            {
                var pc = PropertyChanged;
                if (pc != null)
                {
                    _selected1 = value;
                    pc(this, new PropertyChangedEventArgs("Selected1"));
                }
            }
        }

        private People _selected2;

        public People Selected2
        {
            get
            {
                return _selected2;
            }

            set
            {
                var pc = PropertyChanged;
                if (pc != null)
                {
                    _selected2 = value;
                    pc(this, new PropertyChangedEventArgs("Selected2"));
                }
            }
        }

        private People _selected3;

        public People Selected3
        {
            get
            {
                return _selected3;
            }

            set
            {
                var pc = PropertyChanged;
                if (pc != null)
                {
                    _selected3 = value;
                    pc(this, new PropertyChangedEventArgs("Selected3"));
                }
            }
        }

        private People _selected4;

        public People Selected4
        {
            get
            {
                return _selected4;
            }

            set
            {
                var pc = PropertyChanged;
                if (pc != null)
                {
                    _selected4 = value;
                    pc(this, new PropertyChangedEventArgs("Selected4"));
                }
            }
        }

        private void InitSourceCollection()
        {
            SourceCollection1 = FakeSource();
            SourceCollection2 = FakeSource();
            SourceCollection3 = FakeSource();
            SourceCollection4 = FakeSource();
        }

        private ObservableCollection<People> FakeSource()
        {
            var source = new ObservableCollection<People>();
            source.Add(new People 
            { 
                Name = "people A",
                Age = 23
            });

            source.Add(new People
            {
                Name = "people B",
                Age = 24
            });

            source.Add(new People
            {
                Name = "people C",
                Age = 25
            });

            source.Add(new People
            {
                Name = "people D",
                Age = 26
            });

            source.Add(new People
            {
                Name = "people E",
                Age = 27
            });

            source.Add(new People
            {
                Name = "people F",
                Age = 28
            });

            source.Add(new People
            {
                Name = "people G",
                Age = 29
            });

            source.Add(new People
            {
                Name = "people H",
                Age = 30
            });

            source.Add(new People
            {
                Name = "people I",
                Age = 31
            });

            source.Add(new People
            {
                Name = "people J",
                Age = 32
            });

            source.Add(new People
            {
                Name = "people K",
                Age = 33
            });

            return source;
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}