Sibelius 6th and 7th symphonies good too. Also 3rd.
His tone poems En Saga, The Oceanides, Nightride and Sunrise, Pohjola's Daughter and Tapiola also mini masterpieces.
I've loved Sibelius's music for years but still find the 4th symphony very difficult to understand... so well done!
However... other composers to follow from Sibelius is a very challenging question. His music is very individual and although he was a massive influence over later composers he is quite unique.
Vaughan Williams 3rd, 4th, 5th and 6th symphonies are possibilities. Maybe Carl Nielsen 3rd,4th and 5th symphonies too - written arround same time as Sibelius later works.
Might also be worth trying Shostakovich 6th symphony... first movement is very still and cold, similar atmosphere as Sibelius 4th. I'm sure Shostakovich was thinking of Sibelius when he wrote it. 2nd and 3rd movements are grimly ironic fast movements, so a bit different.
Going back in time from Sibelius might be worth trying.. standard Newbie fare is Tchaikovsky - especially 4th, 5th and (greatest of all) 6th symphony, but some might find it too ardently romantic. Same applies to Rachmaninov, but 2nd, 3rd &,4th piano concerti and 2nd and 3rd symphonies all worthwhile.
Dvorak and Schumann symphonies are worth investigating too. It might be an idea to get hold ofa classical compilation, see what you like on that, then try the complete works based on that. Have fun, and let us know how it goes.