You can reach 4.2 with different mix of stats/resistances/skills so you can have a more reasonable answer if you give the stats.
For example, you can get a 4.2 dragon with max stats but 100 wrestling and 65 physical resistance which is not good for damage output (misses too much) and not good for tanking (won't dodge and will take a lot of physical damage).
If you take the metrics of the pet power calculator you have to look at 4.6+ to be almost certain of what you get.
Otherwise if it has good physical / energy resistance (fire is usually always high enough) and high wrestling then it's definitely a good fighter - hits more often and receives less damage.
Hp are of relative importance.
How much can be worth depends entirely on the buyer (once i sold a spellweaving book for 3 millions because the buyer was full of money and he wanted it asap but the item itself wasn't really worth more than 100k) but you have to consider that all tamers have a more or less good greater dragon: it may take a couple of hours but it is not hard to get a greater dragon, so do not expect big money (maybe 10 millions for a king of the ring and 500k-1 million for a very good one).