Trying to bottle feed

[McIntosh2]

Hi. My baby is nearly 4 months now and since birth he has happily breast and bottle fed. Recently though he has rejected the bottle. I've offered a different test but he just won't take it, no matter how hungry he seems. As soon as I offer the breast he takes it frantically suggesting he is hungry. I go back to work in the next couple months and am really worried what to do if he continues to refuse anything but breast.
Any help or advice really appreciated