You've got a couple of possibilities.

1. Use magnets to push and pull the swimsuit in place. Takes a lot of fiddling, probably several hours. Costs: $0

2. Use a modeling application to change the swimsuit so that it fits perfectly. Takes a lot of fiddling too, several hours at least. Will get you a perfect fit if you keep at it long enough and persistently enough. Costs: $0 for a free modeler (Wings, Blender) to $15000 (Maya Unlimited, Houdini).

3. Buy a magnet set for V4. There's several at DAZ. I've used the Netherworks magnet set myself, it's pretty good. Main advantage: fast solution (minutes), results are pretty good, and the magnet set  can be used on all V4 clothes. Costs: around $10 per magnet set.

4. Buy Dimension3D's Morphing Clothes. It'll generate a matching morph in the swimsuit. Reputedly very good. Main advantage: usable on all figures, not only V4. Costs: $20.

5. Buy The Tailor at DAZ. Similar to Morphing Clothes. Can't recommend it though, the morphs it generates are of mediocre quality at best.

6. Buy Wardrobe Wizard by PhilC. More expensive than Morphing Clothes, but also more versatile - it can be used to convert that V4 swimsuit to Aiko 3, for example. With a little bit of trying results can be very good.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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