In the case of heat metal the better effect is the disadvantage it gives to the victim. 4d8 is not much. A lvl5 martial does this with a rusty one handed weapon, without a spell slot, two less levels, and I resource whatsoever.
And then most lvl4 spells are much more powerful or much more useful.
Multiclassing before lvl5 is usually a bad idea. You can twist the problem how you like, that’s how the game is designed.
Take a paladin 4/fithter 3, and any smite will do much, much more than your heat metal. A champion 3/berzerker 4 (you must be really stupid to do that) will still do much, much more than your heat metal. And he will be able to do it as many time as you will.
Sure, 4d8 in itself is not much, of course. 4d8 per round as a bonus action for 10 rounds with a single spell slot, however, is a lot, especially at a level where a paladin only has second level smite slots. Which is why upcasting heat metal can be extremely powerful. (And it gives disadvantage for those 10 rounds, and there’s no saving throw or attack roll involved. If you’re wearing metal armor, it will fuck you up. And if you happen to be resistant or immune to fire, well, that bard 3 / sorc 4 might have transmuted spell.)
In the case of heat metal the better effect is the disadvantage it gives to the victim. 4d8 is not much. A lvl5 martial does this with a rusty one handed weapon, without a spell slot, two less levels, and I resource whatsoever.
And then most lvl4 spells are much more powerful or much more useful.
Multiclassing before lvl5 is usually a bad idea. You can twist the problem how you like, that’s how the game is designed.
Take a paladin 4/fithter 3, and any smite will do much, much more than your heat metal. A champion 3/berzerker 4 (you must be really stupid to do that) will still do much, much more than your heat metal. And he will be able to do it as many time as you will.
Read the spell description.
Sure, 4d8 in itself is not much, of course. 4d8 per round as a bonus action for 10 rounds with a single spell slot, however, is a lot, especially at a level where a paladin only has second level smite slots. Which is why upcasting heat metal can be extremely powerful. (And it gives disadvantage for those 10 rounds, and there’s no saving throw or attack roll involved. If you’re wearing metal armor, it will fuck you up. And if you happen to be resistant or immune to fire, well, that bard 3 / sorc 4 might have transmuted spell.)