I have no clue what is the resolution of your monitors because 1440p is a family of many resolutions. details: https://en.wikipedia.org/wiki/1440p
Anyway, buying another 1440p monitor is a waste of money. Buying higher resolution than 4K (a.k.a. UHD = 3840x2160 pixels) is also a waste of money. You should buy 4K monitor. The only question is optimum size. The optimum size depends on a distance between the monitor and your nose. The closer, the better. If you use a laptop, you should put it on a stand (to save your neck), and measure horizontal distance from your nose to far end of the stand. This distance is roughly the distance to your monitor. If you use desktop computer, measure the width of your keyboard - this is roughly minimum distance to your monitor.
The figure of merit is angular size of the pixels that are seen by your eyes. The angular size should be at least as big as it is for a laptop. For example, my laptop has 15,6 inch LCD and 1920x1080 resolution, so its pixel density is 1080 pixels / 195 mm = 5.5 pixels/mm. My nose is about 350 mm from my laptop’s LCD, so the angular size of one pixel is 1/(5.5*350) = 0.0005 radian. My monitor has 27 inch LCD and 3840x2160 resolution, so its pixel density is 2160 pixels / 336 mm = 6.4 pixels/mm.
My monitor has higher pixel density (more dots per inch, if you are not metric person), than my laptop, which means that it should be closer to my nose than my laptop. This is not feasible, so my monitor is too small. If you use a laptop, the minimum diagonal size of a 4K monitor is about 40 inches. If you use a desktop computer, the minimum diagonal size of a 4K monitor is about 27 inches.
best laptop stands: https://thewirecutter.com/reviews/best-laptop-stands/
best 4K monitors: https://thewirecutter.com/reviews/best-4k-monitors/