good day, nisachar,
new turing cards like 2080ti and TitanRTX are pretty thirsty and also rather toasty as an effect. That's the reason why NVIDIA redesigned their cooler and went from blower type they had before to new one, just to be able to remove all the heat effectively.
the fact that gaming cards do not have dual slot NVLINK only triple and quad also points to the fact that You should not use those cards at every second PCIe slot (1, 3, 5, 7 on Your mentioned 7 PCIe slot motherboard). Thus logically, if You plan to have four cards (two NVLINKed pairs) I would go with some server mobo with 11 slots seating cards with extra slots between (1, 4, 7, 10) so between all of those You would have and extra gap for cooler to breath.
why is cooling that important? as mentioned before, new cards are pretty toasty and they need optimal airflow to remove that heat. To give You a bit of perspective, 2080ti is about 10C warmer than the warmest 1080ti on the same watercooled loop (and that's actually why blower cooler equipped 2080tis are louder in aircooled builds). If You fail removing the heat, You will end up with card throttling down and loosing render speed (quad 2080tis perform anywhere between 800-1200 in OB
and while RTX Titan is slightly faster, technically it's more or less the same GPU under the hood, but we do not have too much data on quad T RTX build due to high price tag making build lass popular), not to mention that running daily loads heating up and down would effect life span of GPU in a negative way if You have bad cooling.
There are few solutions in Your case:
* get bigger motherboard if You plan to get 4X cards (what is logical if You care about NVLINK pairing),
* get better coolers - could be some hybrids or simple closed loop options
* build a custom loop (this one is pricey and complex)
* stick with two cards (or three if NVLINK is not a priority)
* last but not least, You can use four cards, just keep in mind that You might end up loosing performance, running cards much more hot than needed and have more noise than needed. system stability during long runs might get compromised as well.