It is the best to work with the modified 4T1 cells expressing luciferase, e.g., 4T1-luc2/GFP. If You add luciferin intraperitoneally then You can observe the tumor cells in vivo with non-invasive bioluminescence imaging using an IVIS Spectrum (Caliper).
Or You can make histological slides and observe tumor cells expressing GFP in a fluorescent light.
It is the best to work with the modified 4T1 cells expressing luciferase, e.g., 4T1-luc2/GFP. If You add luciferin intraperitoneally then You can observe the tumor cells in vivo with non-invasive bioluminescence imaging using an IVIS Spectrum (Caliper).
Or You can make histological slides and observe tumor cells expressing GFP in a fluorescent light.
I have done a lot of study about the metastasis of 4T1 cells expressing both GFP and Luc2 (4T1-GFP-Luc2). 4T1-GFP-Luc2 can be either orthotopically implanted in fat pad (~3-4 weeks to develop mets) or intracardiacally injected. The GFP and Luc on 4T1 of intracardiac injection mets model are pretty stable. You can always check the mets development by injecting with D-luciferin and follow by bioluminescent imaging. However, GFP and luc2 on 4T1 of orthotopical mets model are not very stable especially on Balb/c mice. Some of the tumor cells will lose GFP and Luc during metastasis.
I agree with the above comments, infect the cells with a luciferase-GFP reporter plasmid in order to track the grow of the cells in vivo either by IVIS or IHC.