Your Turn This Digimon gets +3000 DP.
Rulings
Q1: Can I use this card's Main effect to pay 4 cost even when I don't have any level 5 or higher Digimon?
No, you can't. The activation conditions include paying 4 cost and placing this card as 1 of your level 5 or higher Digimon's bottom digivolution card, therefore the effect can't be activated just by paying 4 cost.
Q2: Can I use this card's Main effect to place this card as 1 of my level 5 or higher Digimon's bottom digivolution card and suspend 1 of my opponent's Digimon without attacking?
No, you can't. You must choose to suspend 1 of your opponent's Digimon and attack, or you must choose to not suspend 1 of your opponent's Digimon and not attack.
Q3: If my Digimon with multiples of this card in its digivolution cards would be deleted, can I use this card's All Turns inherited effect to play multiples of Parasitemon?
Yes, you can. However, if this card is played before the effect activates, this card's All Turns inherited effect can no longer be activated.
Data Sources
Lv.6 Parasitemon BT17-050 R Mega | Virus | Parasite [Hand] [Main] By paying 4 cost and placing this card as 1 of your level 5 or higher Digimon's bottom digivolution card, you may suspend 1 of your opponent's Digimon and attack an opponent's Digimon with the Digimon this was placed under. <Alliance> [End of Attack] You may place this Digimon as 1 of your other Digimon's bottom digivolution card. --- Inherited: [All Turns] When an opponent's effect would delete this Digimon, you may play 1 [Parasitemon] from this Digimon's digivolution cards without paying the cost. Inherited: [Your Turn] This Digimon gets +3000 DP.
Lv.6 Parasitemon BT17-050 R Mega | Virus | Parasite [Hand] [Main] By paying 4 cost and placing this card as 1 of your level 5 or higher Digimon's bottom digivolution card, you may suspend 1 of your opponent's Digimon and attack an opponent's Digimon with the Digimon this was placed under. <Alliance> [End of Attack] You may place this Digimon as 1 of your other Digimon's bottom digivolution card. --- Inherited: [All Turns] When an opponent's effect would delete this Digimon, you may play 1 [Parasitemon] from this Digimon's digivolution cards without paying the cost. Inherited: [Your Turn] This Digimon gets +3000 DP.
Lv.6 Parasitemon BT17-050 R Mega | Virus | Parasite [Hand] [Main] By paying 4 cost and placing this card as 1 of your level 5 or higher Digimon's bottom digivolution card, you may suspend 1 of your opponent's Digimon and attack an opponent's Digimon with the Digimon this was placed under. <Alliance> [End of Attack] You may place this Digimon as the bottom digivolution card of 1 of your other Digimon. --- Inherited: [All Turns] When an opponent's effect would delete this Digimon, you may play 1 [Parasitemon] from this Digimon's digivolution cards without paying the cost. Inherited: [Your Turn] This Digimon gets +3000 DP.
Lv.6 パラサイモン BT17-050 R 究極体 | ウィルス種 | 寄生型 ［手札］【メイン】4コスト支払い、このカードをLv.5以上の自分のデジモン1体の進化元の下に置くことで、相手のデジモン1体をレストさせ、置いたデジモンで相手のデジモンにアタックできる。 ≪連携≫ 【アタック終了時】このデジモンを他の自分のデジモン1体の進化元の下に置ける。 --- Inherited: 【お互いのターン】このデジモンが相手の効果で消滅するとき、このデジモンの進化元から、「パラサイモン」1枚をコストを支払わずに登場できる。 Inherited: 【自分のターン】このデジモンをDP+3000。