You will probably enjoy RF3, but there is a fair amount less content in that one and it's not nearly as polished. The crafting mechanics aren't nearly as good as RF4 since RF4 was when they gave crafting an (amazingly awesome) overhaul, it's probably the biggest change. Battle mechanics are mostly the same, but there are far fewer dungeons. Friendship mechanics are mostly similar, courting mechanics are a bit different in that the dating system is much less polished. All the events are in the form of requests in RF3, which is nice in some ways (no RNG). I do think the characters are the best in RF3 (my opinion), YMMV. A lot less polish in general though, stuff you may take for granted in RF4 aren't in RF3. Still definitely worth a play though. You can buy it new from Natsume for $20 USD + S&H. No idea about EU though (it is licensed there).

As for RF1/2/F, the games had big overhauls at every iteration and a lot of things were tried out, RF3 is when things really started to be polished up and the mechanics are far better. Farming is pretty ridiculous in some of the earlier games, stuff like the long term flowers having 100+ days base time to grow, combined with the fact that fertilizer formulas needed to be reapplied each day, and you couldn't buy all the fertilizer in RF1 and 2, made it pretty cumbersome to get some of the flowers; especially bad in RF2 when you couldn't buy Greenifiers -.-. It's not too bad for regular farming of vegetables and short term flowers though. Battle is far weaker, and slower paced in the first half of the series, so if you enjoyed the fast paced combat in RF4 and ToD, you will probably be disappointed in RF1/2/F. Far fewer events in the earlier games too. But there are still a fair amount of character interactions. Feel free to give them a shot, but you really should bring your expectations down if you do. All the games in the US are buyable new for $20 USD from Natsume and XSeed, so it's not exactly bank breaking...