There is one way you can resolve it without changing topology, but I usually avoid suggesting it because it can cause more issues if you're not careful:
1). Never use it on the body. Either separate the garment or just select it only;
2). Each vertex can be influenced by no more than 4 vertex groups, the rest will be deleted by s4s during import. Use Limit total to remove the right ones ( with the lowest values) after every time you use Smooth tool;
3). Some vertex groups ( armpits, sometimes knees and elbows ) should not fade smoothly, it will affect the animation. It is best to either avoid using this tool on these areas of clothing or adjust them individually afterward;
4). It can spread the paint too far, be very careful with it. You can use Clean tool and change Limit to, say, 0,05.
Anyway
- Make sure the mesh has Armature modifier and rig selected. This ensures that weights auto normalize when you edit them in Weight paint;
- In Edit Mode disable Limit selection to visible. Holding Ctrl, LMB-click-and-drag to select the garment partly, then press Ctrl L to select the rest;
- Switch to Weight paint and enable Vertex selection masking for painting;
- adjust the settings, if you need it to expand less or more, adjust Expand/Contract;
- use Limit total with Limit set to 4 and Subset to All groups;
- if it won't work, select each breast vertex group and use Smooth tool with more Expand and Subset set to Active group. Don't forget to use Limit total again;
- since you adjusted weights on the mesh with split edges, don't forget to remove doubles to make sure that vertices that share coordinates also have the same vertex group values.
