I propose:
Lava Golem
4
Artifact Creature - Golem
Modular -- 2
1R: Put a +1/+1 counter on Lava Golem.
Whenever Lava Golem has more than 4 +1/+1 counters, sacrifice it. Lava Golem deals 4 damage to target creature or player.
0/2
This has nice synergy with modular, caps the damage at 4, and keeps the monster from getting too big. It is still reasonably costed as a 2/4 for 4, with two very good abilities. My suggestion also includes giving up on the toughness-decreasing aspect of the card, which is very cool but adds a lot of confusion to a simple design. Maybe change the name

?