Also remember that you can pick up Hydromancy as early as level 4 on your sovereign or a champion, if you go up the Mage path and don't mind taking 1 rank in Prodigy. On a 4-essence tile, that's a 100% bonus to food, or a 125% bonus to food on a 5-essence tile. This means that the 2/3/4 tile is more like a 4/3/3 tile if you're concerned about the food, which is clearly superior to the other options (which are more like 6/3/1 or 5.25/3/2 tiles, if you use the same enchantment on them, without accounting for the Scrying Pool). Also, it really doesn't take that long to get the sovereign to level 4 unless you're really unlucky with monster spawns in your area.
To grow to 5 level city you need 5 food. So in al cases you will be capped. The less food the worse. You can add some more food by spells, so 4 is enough. If you have wild grain near, then 3 is enough. But you can live well without 5 level city. 4 level city is good enough.
This is not true. The population requirements are (starting level - population breakpoint - resulting level):
lvl1 - 50 - lvl2
lvl2 - 150 - lvl3
lvl3 - 300 - lvl4
lvl4 - 600 - lvl5
With a base 25 food per grain, any city with two or more grain on the tile can achieve level 2 without food bonuses, and tiles with 6 or more grain can achieve level 3 without food bonuses. You can get food bonuses from the Garden line of buildings (+15/35/50/60 food per grain, with the 60 food per grain bonus being Empire-only), the Well line of improvements (+25%/50%/50%/50% local food production, Town only), the Grocer line of buildings (+5/10/20/40 faction food per grain, with the +40 food per grain bonus coming from a 1-per-faction building, Town only), the Pier line of buildings (+5/5/10/15 food per grain, with the +15 food per grain coming from a Kingdom-only structure, requires a river), and the Great Mill wonder (+25% food production locally, one per game). You can also get +1 grain through the Nature's Bounty spell from one of the magic techs, or +25% food per essence through the Gentle Rain spell from the Hydromancy trait. As a result, you require a food per grain bonus of at least 120 in order to grow a 5-grain tile into a level 5 settlement, which cannot be done without enchantments or Towns (at best, a non-Town unenchanted city that doesn't have the Great Mill from a faction without a global food bonus can have +95 food per grain [Empire] or +90 food per grain [Kingdom] - 25 base + 10/15 for the upgraded Pier + 60/50 for the upgraded Garden [Empire/Kingdom]). Note that I have given the total bonus for each structure rather than the bonus per upgrade.
Food per grain requirements to get a city to level 2/3/4/5 (Total Grain Yield. Total Food Bonus):
It is certainly harder to get cities built on tiles with lower grain yields all the way up to level 5, but it's far from impossible unless you don't build any Towns; a 4-essence tile which you can upgrade to a 5-essence tile should not have any real trouble meeting these requirements, though it may take longer than the 3-, 4-, 5-, or 6-grain tile to do so. Just going with a Kingdom city without access to Pier-line structures, you can get a food bonus of 150 off of 4 essence (fully upgraded Garden + Gentle Rain), which is enough to get you to level 4 with 2 grain; you can then up the grain yield with Nature's Bounty or, if you're lucky, nearby Wild Grain resources, or build a couple of Grocer line buildings in your Towns. Since the Grocer-line buildings stack with one another, you can get these kinds of numbers simply by having a reasonable number of upgraded Grocers in your empire (specifically, having 11 or more fully-upgraded Grocers will allow you to get any city with 2 or more grain to level 5 if the Garden is fully upgraded and no other food bonuses are present; this requirement goes down to 6 for 3 or more grain, and if we allow for enchantments this number can become much lower, though how much lower is dependent on the site essence yield).
Additionally, while it is somewhat of an exploit, you do not lose the bonus gained from attaining level 5 if you stop meeting the population requirements for that city level. Thus, you can stack as many of the food-boosting enchantments as you can fit (or have) onto a city to get it up to level 5 if you desire to do so, and then remove them once you've attained your goal (note that any population over the food limit will disappear from the game on the next turn if you don't do something with it). Additionally, while I wouldn't say that Consulates in Outposts are exactly great ways to boost city growth, they can help if you want to push the growth up a bit. Just be aware that blowing 30 population and several turns of production or a few hundred gold to get +1 population growth isn't always going to make the city reach the next level any faster.