know the open sights are great for 300-400 yard paper and steel but kinda lacking at shooting critters

A plain, good quality 3x9x40 scope w/med. rings sitting on YHM 1/2 risers on your flat top will do the job. The Nikon scopes are great and won't break the bank. Good Luck, I am looking forward to Ground Hog hunting this spring myself. I use a BM 24" Varmiteer with a 3x9x40 matte Nikon Pro Staff with the above ring and mount combo.

Not quite an M4 but a 16" Carbine.

This is how I set mine up for longer range critters.

Top one for distance!

Exactly like this.

Scope is a 3x12x44 for my 59 yr old eyes.  Even though its a Leapers and only cost $140 incl rings, it's solid and clear after 4,000rds on it.  By no means uber 'tacticool' but adequate for me to see and hit a 2 1/2" x 3" steel plate at 300 yds from a rest.  

The snobs here will tell you it's Chinese junk and maybe it is, made it's not.  I keep ranging it with the target knobs between 100 and 300yds and it returns to zero every time.  

A fellow shooter had his $1,800 Nightforce scope mounted on his 50 BMG bolt gun (the scope is made specifically for that setup) and asked me to help spot his shots to sight it in for him last fall.  His scope lasted a total of 4 rounds before the reticle let loose and the scope was worthless.

On that day and every day since, my scope has performed without a problem.