4T1 cells show more mesenchymal morphology and further, continue to diffusely proliferate, different from the picture attached. You should try to add the Plasmocin (the removal of Micoplasma) into the culture medium.
I am currently growing this cell line. They will initially grow in small clusters later spread out very well in the flask. When you split the cells, make sure they are single cells when you seed in the new flask. You have to pipette up and down couple of times after trypsin and view under microscope. If there is clumps, they form small l clusters.
4T1 cell line is one of the most aggressive tumour cell lines. Strong cell aggregation is the main charectristic of this cell line. So if you want to guarantee the normal cell growth, you might regularly subculture cells(avoid high cell confluency).