I guess a lot depends on the resources that are open to you in your imaginary theatre. Most amateur productions have a range of different backdrops that can be slotted into place to indicate a change of setting. If I were directing my version of this play, I would want a backdrop which clearly indicates the change in scene from Act IV scene 1, which we are told takes place in a Roman house, to Act IV scene 2, which we are told takes place in a Camp near Sardis. The backdrop for the camp would obviously have tents in the background, with maybe watchtowers and weapons and armour piled neatly in various locations to indicate the shift from urban Rome to a military camp that is ready for action and war. Clearly, you would have blue sky with a sun, or perhaps an overcast sky to indicate the battle and tragedy ahead that awaits Brutus and his side.
In Act IV, scene 2, the action shifts from Italy to a battleground in Sardis, Greece. The only stage directions the play's text provides are "Camp near Sardis. Before Brutus's tent." We also learn that Brutus, Lucilius and "the army" enter, as well as Titinius and Pindarus.
Clearly, Brutus's tent would need to be pitched in a prominent location in the middle of the stage, but set far enough back to leave room for the characters to meet in front of it. Bringing in an "army" would be difficult: this could be suggested by having Brutus followed by a group of men armed with swords, spears and shields. The set design could suggest the outdoors with a painted backdrop that showed clouds and sky and below that, the rocky terrain of Greece. I might scatter some papier mâché boulders at the rear of the stage, and if there were room without crowding the stage, mount a small, second tent in the back to suggest an encampment. All of this would signal a move from the confines of urban Rome to a battle camp.