I'm not a frog-person with disproportionately strong legs, nor do I know any in real life, but let me offer some analysis based on many years of experience in medieval-style combat, both individual and melee fights.
The shield: You're describing a shield that, in combat posture, covers most of the frog-man's body. This provides good defensive coverage from the front and can be used to intercept blows coming from front and up. It will be harder to move into a position to protect from range weapons, but effective when it can be used (large area). I haven't done a load-bearing analysis of shield-sized rafts, so I am assuming a shield that's reasonably sturdy, probably metal-rimmed for extra protection.
The spear: I'm assuming you're describing a one-handed weapon here because of what you said about the shield. It's possible to use a spear two-handed while also using a shield, but usually (in my experience) this is effective only with smaller shields, not the larger ones you've described. A one-handed spear is good at thrusting but probably not slashing. Thrusting requires more strength than slashing (as with a sword) to do comparable damage, and may be more hindered by better armor. You might consider adding swords to your frog-men's equipment list.
Spears are quite effective when being charged, particularly in combination with those large shields you've described. When your frog-men fight in groups they should form a shield wall, brace against charges, and maintain formation. In the shield-wall formation they might choose to have second-rank fighters forgo their shields in order to use their spears two-handed (for much-more-effective blows). Sword and shield in the front rank and spear in the second rank is an effective combination.
Posture: Being shorter than one's opponent is often a disadvantage, especially if the taller opponent also has longer reach (as when shorter and taller people fight each other; the taller ones tend to have longer arms). But your frog-men have an important advantage: strong jumps. This suggests a tactic where they lurk out of opponents' weapon range waiting for an opening, then jump, strike, and fall back. I would expect this to be comparable in many ways to modern fencing bouts; viewing such bouts might give you ideas.
Upper body strength is important in melee fighting, but strength does not come only from the upper body. The best fighters I know (and granted, we're talking re-creation here with wooden weapons, not live steel, and people are not actually trying to kill each other) start pushing a blow from the back foot, through the legs and hips, and into the upper body until it becomes a powerful weapon blow. (This is hard to describe in words, sorry.) My point here is that your frog-men, being particularly strong in the legs, probably have the element of surprise the first time they strike; nobody's going to be expecting that much power from such a little guy. They should aim for tactics where they can use that surprise.
Because they can walk upright, they have the ability to trade shield coverage for height advantage on an as-needed basis. Human fighters sometimes try to maintain a crouch for protection too, but we can't do it for extended periods because we're not built for that. Having the ability to choose as conditions dictate should be very powerful.
Aerial attacks: Because of those strong legs they can leap -- you said about 4m up. This gives them the ability to leap and strike while dropping back to the ground. I'll leave it to the physicists to compute how powerful their strike would be -- I would expect this could deliver a pretty solid blow, what with gravity on their side like that. However, unless the low-level magic in your world includes flight, once a frog-man is airborne he has limited control over where he goes. Opponents nimble enough to dodge the blow -- or trip him up as he lands -- will be a real danger. Aerial attacks from surprise, on the other hand, can be very effective; if they can ambush their opponents from cover (or even from lower tree branches!) that first blow is likely to be doozy.