Ditto HV3, I also have a fair bit of V4 content I still want to use and I do like the near but not quite real quality of V4 era mats for some works.
I use the Lali's Bits for V4, they are very good plus you get some useful boob morphs and a lovely Lali character to play with.
Installation is not exactly push button and you need to follow the instructions carefully but worthwhile. I'm not sure if you can apply them to existing characters, I never got that to work but I created a 'base' V4 figure with the bits installed and the Lali character dial at zero. Now I simply apply the other character's shapes and mats to that. Properties Pre-sets can be used to transfer poses, adjustments and so on that you want to keep from existing characters.
As for the clothing: You are never going to get V4 clothing to fit like later generation stuff with higher poly counts, enhanced rigging, auto-fit and so on but there are ways to get good if not great results with the older stuff. First explore the scene tab to see if the item has bones that can be manipulated to suit the pose, check for morph / pose dials and as mentioned above look for handles, these may be off by default but if the item has them there will often be handles on / off pre-sets provided.
There are also many other tricks and technologies that can be very helpful. Deformers can be an easy way to fix little bits of poke through although simply adding a smoothing modifier may be even easier. Mesh Grabber is a great tool for in scene geometry manipulation from simple tweaks to complex shape changes. Also check out dForce, bit of a learning curve and does not work on some items but can achieve very authentic fits, drapes, etc on the figure, furniture or floor. There are loads of vids on You Tube showing what can be done with these tools and how to go about it.
I'm just here by mistake, for a friend, for science... anyway you can't prove nuffin'.