what’s the minimum ehp/s to safely tank l4s?
That is far to open ended of a question for an easy answer. It depends on so many factors.
What kind of ship are you flying? A smaller ship will get a good bit of tank via it’s speed and sig.
How are you running them? A close range brawler might need more tank than a MJD sniper fit.
What are your skills? A well skilled marauder with >1000 DPS won’t need as much tank as it clears the field faster.
That being said, eve survival has all of the missions outlined. Many of the mission details show max incoming DPS estimates.
The rule of thumb to run level 4 missions comfortably is Gank + Tank > 1000. where Gank is dps as measured by pyfa and Tank is your recharge rate, also as measured by pyfa.
More gank generally means you need less tank since you clear the room faster.
For most missions range is the best tank. Most run lv4s in battleships, more then enough tank with resistance and a regen of some kind, armor repair etc. even passive shield regen can work.
If you have the skills then a close up brawler is another story, or T2 or T3 cruisers can be fun for those that have the skill.
depending on your skills at manual flying, what ship you are using and missile skills, you can do most L4’s if you blitz anyway, with 0 tank.
If you’re looking for the exact damage output of a mission when all enemies are in range and hitting you well, you can consult this page to read up on the damage output of individual enemies: http://games.chruker.dk/eve_online/inventory.php?category_id=11
Otherwise, the actual incoming damage is very timid and least in a BS you have plenty of buffer to even work with an undertanked fit, putting aside the option to simply shoot stuff before it enters weapon range.
I use a Marauder most of the time. Just sit back and blast from range doing tons of damage. Clear rooms often with little to no damage. The only issues are missions vs. missle ships. You are going to take some damage, so having some resistance and regen of a sort is required.