90C for 2y? It didn't melt a hole in the floor? I have a panic attack whenever I see anything over 70 or 75 during hot summer days... once you get it all together I would stress test your build with Prime95 to make sure the hardware was not physically damaged by excessive heat
Jokes aside, as was mentioned before a delid is not necessary on a locked CPU, I wouldn't use the stock cooler though. Even on an unlocked CPU a delid is not always necessary unless you want to do some serious OC'ing. Even the i7-7700K, in which intel used toothpaste as TIM between the die and the IHS, runs at perfectly fine temps at stock clock speeds without a delid.
A good cooler is always advisable though. Look at it this way, good coolers are made extremely versatile and compatible with nearly all existing sockets and often they will still be compatible with newer sockets. Something like a Corsair water cooler that costs 100-200$ will last you through several builds and upgrades, it's a long term investment.
Also, when mounting a cooler use this thing
http://www.arcticsilver.com/arcticlean.htm to clean up the TIM, alcohol doesn't do a good enough job.
Don't know anything about Arctic MX-4, but you can use Conductonaut between the IHS and the heat sink too, you just have to be careful since it's conductive so if it gets on the mob or into the socket it will cause a short circuit. But the thing isn't that liquid, I've worked with it when I delided my own i7-7700K. It's pretty viscous still, you just have to be careful. Do at your own risk ofc.
*This is a bit off topic, but honestly, deliding and OC'ing are generally not that super worth it unless your RIG is in the end-cycle of it's life (becoming obsolete) and you want to run it into the ground before upgrading. Without resorting to extreme measures (Liquid Nitrogen cooling for example) you will at best get an extra 10% of your base stock speed, or so... but clock speed isn't everything, that extra 500 mhz you will squeeze out will amount to possibly 5-10 frames in game on average... there are some separate special cases but on average that's about it... on the other hand you get a less stable system, more heat, potentially lower CPU life and a void warranty (might be irrelevant if already expired) so meuh…