Lol... I agree - a new violin will DEFINITELY make all the difference!!!
Seriously... Almost all my students do struggle with 4th finger - but adults more so than children/teenagers.
I don't know whether its because adults are less flexible, or that 4th finger gets used to being pretty lazy and doesn't appreciate being asked to do some work!!! But with regular practice of scales with 4ths instead of open strings, technical exercises aimed at improving 4th finger, and lots of fun relatively easy pieces with 4th fingers in instead of open, you should feel yourself improving. Its also worth looking at your arm position - your elbow being in the right place can help a lot with stretching. Pay attention also to your thumb position, and the shape of your fingers...
I think the reason it takes so long is that you are essentially trying to build up muscle and strength in a weak finger, which will take time... but eventually you will find it much easier! Independence of your 3rd and 4th finger also needs work, as you have found out! They don't like working on their own! so lots of 3rd and 4th finger practice and exercises will help... I find most of my pupils prefer doing 4th finger in a 1 octave A major scale (G and D string) to start with - so 3rd finger is #, and 4th is next to it... you then can extend it to 2 octaves and work on the stretch between 3rd finger D and 4th E on the A string...
If you practice slowly thinking about pitch and position it should get easier?! Hope this helps!