For dual battery set ups, sigelei 150w tc is hard to beat as is the ipv 3li.the ipv 4 is pretty good but some have reported issues with temp control and with the general build quality. I have a smok x cube 2 which pairs perfectly with the tfv4 and is at the cheaper end of the scale but needs a lot of tinkering to get tc working well. All of the above have upgradable firmware. Not many single battery set ups will probably look right with the tfv4 tbh it's a beast of a tank. The ipv d2 and sigelei 75w seem to be the most highly recommended and I'm personally waiting for an ipv d2. Those are the recommendations for user replaceable battery mods. For built in batteries the evic vt or vtmini or istick 40w probably. Remember that you'll probably need an external charger for the dual battery mods as internal chargers seem to be a little hit and miss.