Thermalright Phantom Spirit 120 SE Black CPU Cooler,Dual TL-C12B V2 PWM Fans and Double Towers, 7 Heat Pipes Heatsink Cooler,for AM4/AM5&Intel LGA1851/1700/115X/1200, Computer CPU Cooling,Black

4U Chassis /should/ support up to a 155mm tall CPU cooler, but depending on the case I've found that 154mm is a better limit as 155mm can sometimes scrape the top cover. Either way, depending on your board you should consider coolers like the Thermalright Burst Assassin 120 EVO or ID Cooling SE-225-XT. That is assuming you're using a standard socket (AM4/5 or hLGA1700/1851) If you are using a beefier CPU (270K+, 285K, 7950, or 9950) then I would suggest going for the bigger brothers of these coolers like the Thermalright Phantom Spirit 120 SE or ID Cooling Frozn A620.

How about taking the heatsink off and maybe reapplying thermal paste and remounting the heatsink. Use isopropyl alcohol to clean the previous thermal paste off and try again. Make sure its evenly spread on the surface of the CPU and when you mount the heatsink it should be a pretty tight connection. If you're unsure check youtube videos on how to do so. If you can I'd opt for a fairly a Thermalright Phantom Spirit: https://www.amazon.com/Thermalright-Phantom-TL-C12B-Heatsink-Computer/dp/B0D93R3YY9 for proper cooling. It should fit in your case fine. Again that cpu should be much lower on idle if it is properly installed.