what exactly am i supposed to be doing with a four/five month old? please help! feel like im going crazy

As my mum told me when my first was a baby - you don't need to entertain them! Boredom is good for kids. I kept my sanity by going out for walks a lot. I found time passed so painfully slowly in the house but quickly if we were out. Go out for walks, meet up with friends or family. In the house just do what you'd normally but with baby in tow. It doesn't matter if they grizzle and moan.

All these are great suggestions.
I read to my two several times a day - even when very little because that was my favourite thing. So I was building up the attention span for books so as they got older a bag of small board books would absorb them for longer and longer over time. Also repeating the same book every day - they learn it and it becomes its own reassurance. So This Is The Bear for example - great rhyming simple story was a typical favorite. And they quickly show preference for a familiar story pre speech.
Limiting screen time (was easier pre iPads/smart phones obviously) but I think it teaches a baby to manage low stimulation situations- life/school is full of moments of having to manage your own mind, and out of boredom comes many ideas and independent thought.
Also, lots of singing, music and simple baby instruments, nursery rhymes, the library to get new books, meeting up with people for your own sanity. This stage goes really quickly and very slowly which is a mystery.
I used to sing the 'grandfather clock' when putting them down to signal sleep time. They love a routine.
But you need a routine which suits you.

This Is The Bear by Sarah Hayes - perfect book. Whatever Next by Jill Murphy, goodnight moon by Margaret Wise Brown (fantastic for a younger baby)
The cat in the hat
All the Julia Donaldsons

Sop stressing.

Then get out and about.

Go folk a walk.
Meet someone for lunch.
Go to some Baby groups.
Swimming.

I feel for you OP - I found the 4-6 month mark really difficult. My little boy was so frustrated after 2 mins of anything, and each day felt like a lifetime. I used to describe it as rotating round the stations- play mat, bouncer, jumperoo, bumbo seat. Rinse and repeat. He would be happy for a minute or two and then start whining. He wasn't old enough for most toys, but he also was not happy just laid there doing nothing. He also hated baby wearing so that wasn't an option for us either.

I second what most people are saying about getting out of the house. We went to about 2 classes a week which wasn't a lot. Outside of that I would walk and walk and walk with the pram. It was a chance for some time to myself and a relief from the whining - thankfully he loved being outside and was happy in the pram.

I can't offer much more as I really struggled with this age too, but all I can say is that it's a phase and it does get better. The jumperoo was a life saver for us FYI, it was the one thing that he was happy staying in a bit longer (maybe 5-10 mins rather than the usual 2-3) 🤣

Yep babies are pretty dull. I made sure I gut out if the house most days. Went for a long walk ending up in a cafe. The rest of the time I'd alternate between being focussed on the baby and going about usual household stuff, chatting to the baby (just narrate what you are doing). But yep dullsville and I often caught myself looking at the clock and calculating when it was time gif his bath and bed!
When he becomes a toddler this will disappear as you will spend most of your time making sure he doesn't kill himself by eating/climbing/falling/drawing on the walls etc!

My easiest baby was my third, I think this was mainly because I had two others to deal with (eldest was only 4 when she was born) so I never really had to think of anything to do with her. She just came along wherever we were going, or got plonked on a baby play mat thingy and watched us going about our business around her. I had to get out multiple times a day for drop off and pick up stuff and there was always something going on, so I never really had to actively try to entertain her.

Obviously you can't borrow another couple of kids, but my point is that it's easier if you don't focus on 'entertaining' a small baby and just kind of get on with your day. Walk to places where you usually would drive, it kills time and gives you lots of things to look at and point out to the baby. I could kill a couple of hours with my first just by going to the supermarket because I didn't have a car, so I would walk 40 minutes each way and just chat to him if he was awake.

Changes of scene is everything.

As PP said, don't feel like you have to entertain them. They'll learn a lot by watching and just being with you while you carry on with your day.

Oh OP, I remember this stage so well. Felt like I was constantly wondering what I could do next.

In the end I discovered he loved sitting in front of the washing machine in his bouncer so made sure to put a wash on each day! That plus getting out meant I was killing a few hours between nap times and finally he got more capable (and less boring!)

Play mats are nice with hanging toys on them. Take them out for walks in the buggie to get fresh air. Do your usual chores with them in a bouncy chair. Find a nice park with a nice cafe. Meet other Mums and gossip.

I have a 4.5 month old. He is a relatively easy baby (my older DS was not). I tend to lay him down on the floor with toys around him and he rolls onto his side to look, or onto his tummy to reach and mouth them. He lays in a bouncer and watches me play with his brother. He goes in a jumperoo and thrashes about. He talks/shouts to himself. He sits in a Bumbo and hits dangling toys. He sits on me and we do nursery rhymes or just "chat". He splashes about in the bath. He sits in the corner of the sofa. He lays on his back and chews his own feet. He does watch a bit of TV when his brother is (usually Cocomelon, PJ Masks, or Cars).

The Baby Club on CBeebies is great for ideas (it's on iPlayer).

Are they old enough for a jumperoo? I can’t remember, but my son absolutely loved that.

Also, a playmat with a piano at the end for him to kick was a big hit.

I think walking round the house pointing out things is great, and singing nursery rhymes is great for them. Baby groups and things are probably more for you at that age but there’s nothing wrong with that.