well for me, doing 4U math enhanced my problem solving abilities (I reckon the harder 3U topics had the most impact on that), I was able to do most of the last questions of the 3U paper that i couldn't do before
Ext 1 is crucial for Ext 2 if your aiming for the high bands because it teaches you a special type of thought process technique that is not available in 2u but is available in 4u and this thought process is really what takes you to the next level. But if your willing to stay shallow in 4u than 3u is not much of a problem really.